diff --git a/src/emu/ovnisort.c b/src/emu/ovnisort.c
index a439552..ac15673 100644
--- a/src/emu/ovnisort.c
+++ b/src/emu/ovnisort.c
@@ -74,6 +74,21 @@ ring_add(struct ring *r, struct ovni_ev *ev)
 		r->head = 0;
 }
 
+static void
+ring_check(struct ring *r, long long from)
+{
+	uint64_t last_clock = 0;
+	long long start = from != -1 ? from : r->head;
+	for (long long i = start; i != r->tail; i = i + 1 >= r->size ? 0 : i + 1) {
+		uint64_t clock = r->ev[i]->header.clock;
+		if (clock < last_clock) {
+			die("ring not sorted at i=%lld, last_clock=%"PRIu64" clock=%"PRIu64 ,
+					i, last_clock, clock);
+		}
+		last_clock = clock;
+	}
+}
+
 static ssize_t
 find_destination(struct ring *r, uint64_t clock)
 {
@@ -289,6 +304,7 @@ execute_sort_plan(struct sortplan *sp)
 
 	/* Set the pointer to the first event that may be affected */
 	struct ovni_ev *first = sp->r->ev[i0];
+	long long dirty = i0;
 
 	/* Allocate a working buffer */
 	uintptr_t bufsize = (uintptr_t) sp->next - (uintptr_t) first;
@@ -306,6 +322,10 @@ execute_sort_plan(struct sortplan *sp)
 
 	free(buf);
 
+	/* Invariant: The ring buffer is always sorted here. Check from the
+	 * dirty position onwards, so we avoid scanning all events. */
+	ring_check(sp->r, dirty);
+
 	return 0;
 }
 
@@ -366,6 +386,12 @@ stream_winsort(struct stream *stream, struct ring *r)
 			}
 		}
 
+		/* FIXME: After executing a sort plan, we may have pointers that
+		 * now point to garbage, as the events have moved. We can test
+		 * this by first sorting a given region so that it displaces the
+		 * offsets of the end marker, and then injecting events that are
+		 * inside the first unsorted region. Hopefully, trying to access
+		 * the clock of the end marker will now read garbage. */
 		ring_add(r, ev);
 	}