diff --git a/src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go b/src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go
index c3a8c624e..ee61cf5aa 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go
@@ -6,6 +6,7 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
+// updateLatestEvents updates the list of latest events for this room.
 func updateLatestEvents(
 	db RoomEventDatabase, roomNID types.RoomNID, stateAtEvent types.StateAtEvent, event gomatrixserverlib.Event,
 ) error {
@@ -14,8 +15,15 @@ func updateLatestEvents(
 		return err
 	}
 	defer func() {
-		// Commit if there wasn't an error.
-		updater.Close(err == nil)
+		if err == nil {
+			// Commit if there wasn't an error.
+			// Set the returned err value if we encounter an error committing.
+			err = updater.Close(true)
+		} else {
+			// Ignore any error we get rolling back since we don't want to
+			// clobber the current error
+			updater.Close(false)
+		}
 	}()
 
 	var prevEvents []gomatrixserverlib.EventReference
@@ -54,11 +62,11 @@ func updateLatestEvents(
 		})
 	}
 
-	err = updater.SetLatestEvents(roomNID, newLatest)
-	if err != nil {
+	if err = updater.SetLatestEvents(roomNID, newLatest); err != nil {
 		return err
 	}
 
-	// The deferred fires and the transaction closes.
-	return nil
+	// The err should be nil at this point.
+	// But when we call Close in the defer above it might set an error here.
+	return err
 }
diff --git a/src/github.com/matrix-org/dendrite/roomserver/storage/previous_events_table.go b/src/github.com/matrix-org/dendrite/roomserver/storage/previous_events_table.go
index e7d2d039e..2d081f936 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/storage/previous_events_table.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/storage/previous_events_table.go
@@ -21,7 +21,11 @@ CREATE TABLE IF NOT EXISTS previous_events (
 );
 `
 
-// Insert an entry of the prev event or if the prev_event doesn't exist.
+// Insert an entry into the previous_events table.
+// The there is already an entry indicating that an event references that previous event then
+// add the event NID to the list to indicate that this event references that previous event as well.
+// This should only be modified while holding a "FOR UPDATE" lock on the row in the rooms table for this room.
+// The lock is necessary to avoid data races when checking whether an event is already referenced by another event.
 const insertPreviousEventSQL = "" +
 	"INSERT INTO previous_events VALUES" +
 	" (previous_event_id, previous_reference_sha256, event_nids)" +
@@ -30,6 +34,8 @@ const insertPreviousEventSQL = "" +
 	" DO UPDATE SET event_nids = array_append(previous_events.event_nids, $3)" +
 	" WHERE $3 != ANY(previous_events.event_nids)"
 
+// Check if the event is referenced by another event in the table.
+// This should only be done while holding a "FOR UPDATE" lock on the row in the rooms table for this room.
 const selectPreviousEventExistsSQL = "" +
 	"SELECT 1 FROM previous_events" +
 	" WHERE previous_event_id = $1 AND previous_reference_sha256 = $2"