diff --git a/src/emu/bay.c b/src/emu/bay.c
index 1fd5957..a8f968b 100644
--- a/src/emu/bay.c
+++ b/src/emu/bay.c
@@ -106,7 +106,7 @@ bay_init(struct bay *bay)
 static int
 propagate_chan(struct bay_chan *bchan)
 {
-	dbg("propagating dirty channel %p\n", (void *) bchan);
+	dbg("- propagating dirty channel %s\n", bchan->chan->name);
 
 	struct bay_cb *cur = NULL;
 	struct bay_cb *tmp = NULL;
@@ -123,7 +123,7 @@ propagate_chan(struct bay_chan *bchan)
 int
 bay_propagate(struct bay *bay)
 {
-	dbg("propagating channels\n");
+	dbg("-- propagating channels begins\n");
 	struct bay_chan *cur = NULL;
 	struct bay_chan *tmp = NULL;
 	DL_FOREACH_SAFE(bay->dirty, cur, tmp) {
@@ -134,8 +134,20 @@ bay_propagate(struct bay *bay)
 		}
 	}
 
+	/* Flush channels after running all the dirty callbacks, so we
+	 * capture any potential double write when running the
+	 * callbacks */
+	DL_FOREACH_SAFE(bay->dirty, cur, tmp) {
+		if (chan_flush(cur->chan) != 0) {
+			err("bay_propagate: chan_flush failed\n");
+			return -1;
+		}
+	}
+
 	bay->dirty = NULL;
 
+	dbg("-- propagating channels ends\n");
+
 	return 0;
 }
 
diff --git a/src/emu/chan.c b/src/emu/chan.c
index 63138c3..638aaf4 100644
--- a/src/emu/chan.c
+++ b/src/emu/chan.c
@@ -36,6 +36,10 @@ static int
 set_dirty(struct chan *chan)
 {
 	if (chan->is_dirty) {
+		/* Already dirty and allowed, no need to do anything */
+		if (chan->allow_dirty_write)
+			return 0;
+
 		err("channel %s already dirty\n", chan->name);
 		return -1;
 	}
@@ -44,7 +48,8 @@ set_dirty(struct chan *chan)
 
 	if (chan->dirty_cb != NULL) {
 		if (chan->dirty_cb(chan, chan->dirty_arg) != 0) {
-			err("dirty callback failed\n");
+			err("set_dirty %s: dirty callback failed\n",
+					chan->name);
 			return -1;
 		}
 	}
@@ -60,7 +65,8 @@ check_duplicates(struct chan *chan, struct value *v)
 	//	return 0;
 
 	if (value_is_equal(&chan->last_value, v)) {
-		err("check_duplicates: same value as last_value\n");
+		err("check_duplicates %s: same value as last_value\n",
+				chan->name);
 		return -1;
 	}
 
@@ -71,27 +77,30 @@ int
 chan_set(struct chan *chan, struct value value)
 {
 	if (chan->type != CHAN_SINGLE) {
-		err("chan_set: cannot set on non-single channel\n");
+		err("chan_set %s: cannot set on non-single channel\n",
+				chan->name);
 		return -1;
 	}
 
-	if (chan->is_dirty) {
-		err("chan_set: cannot modify dirty channel\n");
+	if (chan->is_dirty && !chan->allow_dirty_write) {
+		err("chan_set %s: cannot modify dirty channel\n",
+				chan->name);
 		return -1;
 	}
 
 	if (check_duplicates(chan, &value) != 0) {
-		err("chan_set: cannot set a duplicated value\n");
+		err("chan_set %s: cannot set a duplicated value\n",
+				chan->name);
 		return -1;
 	}
 
 	char buf[128];
-	dbg("chan_set: channel %p sets value %s\n", (void *) chan,
-			value_str(value, buf));
+	dbg("chan_set %s: sets value to %s\n",
+			chan->name, value_str(value, buf));
 	chan->data.value = value;
 
 	if (set_dirty(chan) != 0) {
-		err("chan_set: set_dirty failed\n");
+		err("chan_set %s: set_dirty failed\n", chan->name);
 		return -1;
 	}
 
@@ -108,31 +117,34 @@ int
 chan_push(struct chan *chan, struct value value)
 {
 	if (chan->type != CHAN_STACK) {
-		err("chan_push: cannot push on non-stack channel\n");
+		err("chan_push %s: cannot push on non-stack channel\n",
+				chan->name);
 		return -1;
 	}
 
-	if (chan->is_dirty) {
-		err("chan_push: cannot modify dirty channel\n");
+	if (chan->is_dirty && !chan->allow_dirty_write) {
+		err("chan_push %s: cannot modify dirty channel\n",
+				chan->name);
 		return -1;
 	}
 
 	if (check_duplicates(chan, &value) != 0) {
-		err("chan_push: cannot push a duplicated value\n");
+		err("chan_push %s: cannot push a duplicated value\n",
+				chan->name);
 		return -1;
 	}
 
 	struct chan_stack *stack = &chan->data.stack;
 
 	if (stack->n >= MAX_CHAN_STACK) {
-		err("chan_push: channel stack full\n");
-		abort();
+		err("chan_push %s: channel stack full\n", chan->name);
+		return -1;
 	}
 
 	stack->values[stack->n++] = value;
 
 	if (set_dirty(chan) != 0) {
-		err("chan_set: set_dirty failed\n");
+		err("chan_push %s: set_dirty failed\n", chan->name);
 		return -1;
 	}
 
@@ -150,34 +162,36 @@ int
 chan_pop(struct chan *chan, struct value evalue)
 {
 	if (chan->type != CHAN_STACK) {
-		err("chan_pop: cannot pop on non-stack channel\n");
+		err("chan_pop %s: cannot pop on non-stack channel\n",
+				chan->name);
 		return -1;
 	}
 
-	if (chan->is_dirty) {
-		err("chan_pop: cannot modify dirty channel\n");
+	if (chan->is_dirty && !chan->allow_dirty_write) {
+		err("chan_pop %s: cannot modify dirty channel\n",
+				chan->name);
 		return -1;
 	}
 
 	struct chan_stack *stack = &chan->data.stack;
 
 	if (stack->n <= 0) {
-		err("chan_pop: channel stack empty\n");
+		err("chan_pop %s: channel stack empty\n", chan->name);
 		return -1;
 	}
 
 	struct value *value = &stack->values[stack->n - 1];
 
 	if (!value_is_equal(value, &evalue)) {
-		err("chan_pop: unexpected value %ld (expected %ld)\n",
-				value->i, evalue.i);
+		err("chan_pop %s: unexpected value %ld (expected %ld)\n",
+				chan->name, value->i, evalue.i);
 		return -1;
 	}
 
 	stack->n--;
 
 	if (set_dirty(chan) != 0) {
-		err("chan_set: set_dirty failed\n");
+		err("chan_pop %s: set_dirty failed\n", chan->name);
 		return -1;
 	}
 
@@ -194,7 +208,7 @@ get_value(struct chan *chan, struct value *value)
 
 	struct chan_stack *stack = &chan->data.stack;
 	if (stack->n <= 0) {
-		err("get_value: channel stack empty\n");
+		err("get_value %s: channel stack empty\n", chan->name);
 		return -1;
 	}
 
@@ -208,9 +222,29 @@ int
 chan_read(struct chan *chan, struct value *value)
 {
 	if (get_value(chan, value) != 0) {
-		err("chan_read: get_value failed\n");
+		err("chan_read %s: get_value failed\n", chan->name);
 		return -1;
 	}
 
 	return 0;
 }
+
+/** Remove the dirty state */
+int
+chan_flush(struct chan *chan)
+{
+	if (!chan->is_dirty) {
+		err("chan_flush %s: channel is not dirty\n", chan->name);
+		return -1;
+	}
+
+	chan->is_dirty = 0;
+
+	return 0;
+}
+
+void
+chan_dirty_write(struct chan *chan, int allow)
+{
+	chan->allow_dirty_write = allow;
+}
diff --git a/src/emu/chan.h b/src/emu/chan.h
index 9659c2f..47bd839 100644
--- a/src/emu/chan.h
+++ b/src/emu/chan.h
@@ -35,6 +35,7 @@ struct chan {
 	enum chan_type type;
 	union chan_data data;
 	int is_dirty;
+	int allow_dirty_write;
 	struct value err_value;
 	struct value last_value;
 	chan_cb_t dirty_cb;
@@ -51,6 +52,8 @@ int chan_push(struct chan *chan, struct value value);
 int chan_pop(struct chan *chan, struct value expected);
 int chan_read(struct chan *chan, struct value *value);
 enum chan_type chan_get_type(struct chan *chan);
+int chan_flush(struct chan *chan);
+void chan_dirty_write(struct chan *chan, int allow);
 
 /* Called when it becomes dirty */
 void chan_set_dirty_cb(struct chan *chan, chan_cb_t func, void *arg);
diff --git a/src/emu/mux.c b/src/emu/mux.c
index 15c990c..0c46cb6 100644
--- a/src/emu/mux.c
+++ b/src/emu/mux.c
@@ -64,7 +64,8 @@ cb_select(struct chan *sel_chan, void *ptr)
 		return -1;
 	}
 
-	dbg("selecting mux input %p\n", (void *) input);
+	dbg("mux selects input key=%s chan=%s\n",
+			value_str(sel_value, buf), input->chan->name);
 
 	/* Set to null by default */
 	struct value out_value = value_null();
@@ -102,8 +103,11 @@ cb_input(struct chan *in_chan, void *ptr)
 	}
 
 	/* Nothing to do, the input is not selected */
-	if (input == NULL || input->chan != in_chan)
+	if (input == NULL || input->chan != in_chan) {
+		dbg("mux: input channel %s changed but not selected\n",
+				in_chan->name);
 		return 0;
+	}
 
 	dbg("selected mux input %s changed\n", in_chan->name);
 
@@ -135,6 +139,11 @@ mux_init(struct mux *mux,
 		return -1;
 	}
 
+	/* The output channel must accept multiple writes in the same
+	 * propagation phase, as we may write to the input and select
+	 * channel at the same time. */
+	chan_dirty_write(output, 1);
+
 	memset(mux, 0, sizeof(struct mux_input));
 	mux->select = select;
 	mux->output = output;
diff --git a/test/unit/chan.c b/test/unit/chan.c
index 3f8bf00..6e17f82 100644
--- a/test/unit/chan.c
+++ b/test/unit/chan.c
@@ -2,7 +2,46 @@
 #include "common.h"
 
 static void
-check_single(void)
+test_dirty(void)
+{
+	struct chan chan;
+	chan_init(&chan, CHAN_SINGLE, "testchan");
+
+	if (chan_set(&chan, value_int64(1)) != 0)
+		die("chan_set failed\n");
+
+	/* Now channel is dirty */
+
+	if (chan_set(&chan, value_int64(2)) == 0)
+		die("chan_set didn't fail\n");
+
+	if (chan_flush(&chan) != 0)
+		die("chan_flush failed\n");
+
+	chan_dirty_write(&chan, 1);
+
+	if (chan_set(&chan, value_int64(3)) != 0)
+		die("chan_set failed\n");
+
+	/* Now is dirty, but it should allow multiple writes */
+	if (chan_set(&chan, value_int64(4)) != 0)
+		die("chan_set failed\n");
+
+	struct value value;
+	struct value four = value_int64(4);
+
+	if (chan_read(&chan, &value) != 0)
+		die("chan_read failed\n");
+
+	if (!value_is_equal(&value, &four))
+		die("chan_read returned unexpected value\n");
+
+	if (chan_flush(&chan) != 0)
+		die("chan_flush failed\n");
+}
+
+static void
+test_single(void)
 {
 	struct chan chan;
 	struct value one = { .type = VALUE_INT64, .i = 1 };
@@ -33,7 +72,7 @@ check_single(void)
 }
 
 //static void
-//check_stack(void)
+//test_stack(void)
 //{
 //	struct chan chan;
 //
@@ -91,7 +130,8 @@ check_single(void)
 
 int main(void)
 {
-	check_single();
-	//check_stack();
+	test_single();
+	test_dirty();
+	//test_stack();
 	return 0;
 }
diff --git a/test/unit/mux.c b/test/unit/mux.c
index af1bc20..862b995 100644
--- a/test/unit/mux.c
+++ b/test/unit/mux.c
@@ -1,7 +1,7 @@
 #include "emu/mux.h"
 #include "common.h"
 
-#define N 4
+#define N 6
 
 //static int
 //select_active_thread(struct mux *mux,
@@ -39,6 +39,132 @@
 //	return 0;
 //}
 
+static void
+check_output(struct mux *mux, struct value expected)
+{
+	struct value out_value = value_null();
+	if (chan_read(mux->output, &out_value) != 0)
+		die("chan_read() failed for output channel\n");
+
+	if (!value_is_equal(&out_value, &expected)) {
+		char buf1[128];
+		char buf2[128];
+		die("unexpected value found %s in output (expected %s)\n",
+				value_str(out_value, buf1),
+				value_str(expected, buf2));
+	}
+
+	err("----- output ok -----\n");
+}
+
+static void
+test_select(struct mux *mux, int key)
+{
+	if (chan_set(mux->select, value_int64(key)) != 0)
+		die("chan_set failed\n");
+
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	check_output(mux, value_int64(1000 + key));
+}
+
+static void
+test_input(struct mux *mux, int key)
+{
+	/* Set the select channel to the selected key */
+	test_select(mux, key);
+
+	int new_value = 2000 + key;
+
+	/* Then change that channel */
+	struct mux_input *mi = mux_find_input(mux, value_int64(key));
+	if (mi == NULL)
+		die("mux_find_input failed to locate input %d\n", key);
+
+	if (chan_set(mi->chan, value_int64(new_value)) != 0)
+		die("chan_set failed\n");
+
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	check_output(mux, value_int64(new_value));
+}
+
+static void
+test_select_and_input(struct mux *mux, int key)
+{
+	/* Set the select channel to the selected key, but don't
+	 * propagate the changes yet */
+	if (chan_set(mux->select, value_int64(key)) != 0)
+		die("chan_set failed\n");
+
+	int new_value = 2000 + key;
+
+	/* Also change that channel */
+	struct mux_input *mi = mux_find_input(mux, value_int64(key));
+	if (mi == NULL)
+		die("mux_find_input failed to locate input %d\n", key);
+
+	if (chan_set(mi->chan, value_int64(new_value)) != 0)
+		die("chan_set failed\n");
+
+	/* Write twice to the output */
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	check_output(mux, value_int64(new_value));
+}
+
+static void
+test_input_and_select(struct mux *mux, int key)
+{
+	int new_value = 2000 + key;
+
+	/* First change the input */
+	struct mux_input *mi = mux_find_input(mux, value_int64(key));
+	if (mi == NULL)
+		die("mux_find_input failed to locate input %d\n", key);
+
+	if (chan_set(mi->chan, value_int64(new_value)) != 0)
+		die("chan_set failed\n");
+
+	/* Then change select */
+	if (chan_set(mux->select, value_int64(key)) != 0)
+		die("chan_set failed\n");
+
+	/* Write twice to the output */
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	check_output(mux, value_int64(new_value));
+}
+
+static void
+test_mid_propagate(struct mux *mux, int key)
+{
+	int new_value = 2000 + key;
+
+	struct mux_input *mi = mux_find_input(mux, value_int64(key));
+	if (mi == NULL)
+		die("mux_find_input failed to locate input %d\n", key);
+
+	if (chan_set(mi->chan, value_int64(new_value)) != 0)
+		die("chan_set failed\n");
+
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	if (chan_set(mux->select, value_int64(key)) != 0)
+		die("chan_set failed\n");
+
+	if (bay_propagate(mux->bay) != 0)
+		die("bay_propagate failed\n");
+
+	check_output(mux, value_int64(new_value));
+}
+
+
 int
 main(void)
 {
@@ -81,27 +207,11 @@ main(void)
 	if (bay_propagate(&bay) != 0)
 		die("bay_propagate failed\n");
 
-
-	/* Change select channel */
-	if (chan_set(&select, value_int64(2)) != 0)
-		die("chan_set failed\n");
-
-	/* Propagate values and call the callbacks */
-	if (bay_propagate(&bay) != 0)
-		die("bay_propagate failed\n");
-
-	struct value out_value = value_null();
-	if (chan_read(&output, &out_value) != 0)
-		die("chan_read() failed for output channel\n");
-
-	struct value expected_value = value_int64(1002);
-	if (!value_is_equal(&out_value, &expected_value)) {
-		char buf1[128];
-		char buf2[128];
-		die("unexpected value found %s in output (expected %s)\n",
-				value_str(out_value, buf1),
-				value_str(expected_value, buf2));
-	}
+	test_select(&mux, 1);
+	test_input(&mux, 2);
+	test_select_and_input(&mux, 3);
+	test_input_and_select(&mux, 4);
+	test_mid_propagate(&mux, 5);
 
 	err("OK\n");