diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 2b2828266..aacc8a919 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -345,7 +345,7 @@ func (t *txnReq) processTransaction(ctx context.Context) (*gomatrixserverlib.Res
 	}
 
 	if c := len(results); c > 0 {
-		util.GetLogger(ctx).Infof("Processed %d PDUs from transaction %q", c, t.TransactionID)
+		util.GetLogger(ctx).Infof("Processed %d PDUs from %v in transaction %q", c, t.Origin, t.TransactionID)
 	}
 	return &gomatrixserverlib.RespSend{PDUs: results}, nil
 }
diff --git a/roomserver/storage/postgres/deltas/2021041615092700_state_blocks_refactor.go b/roomserver/storage/postgres/deltas/2021041615092700_state_blocks_refactor.go
index 6b72de51a..06740dc8b 100644
--- a/roomserver/storage/postgres/deltas/2021041615092700_state_blocks_refactor.go
+++ b/roomserver/storage/postgres/deltas/2021041615092700_state_blocks_refactor.go
@@ -220,7 +220,6 @@ func UpStateBlocksRefactor(tx *sql.Tx) error {
 			index := stateSnapshotData{snapshot.StateSnapshotNID, snapshot.RoomNID}
 			newsnapshots[index] = append(newsnapshots[index], blocknid)
 		}
-
 		for snapshotdata, newblocks := range newsnapshots {
 			var newblocksarray pq.Int64Array
 			for _, b := range newblocks {
@@ -229,11 +228,11 @@ func UpStateBlocksRefactor(tx *sql.Tx) error {
 
 			var newNID types.StateSnapshotNID
 			err = tx.QueryRow(`
-				INSERT INTO roomserver_state_snapshots (state_snapshot_hash, room_nid, state_block_nids)
-					VALUES ($1, $2, $3)
-					ON CONFLICT (state_snapshot_hash) DO UPDATE SET room_nid=$2
-					RETURNING state_snapshot_nid
-			`, newblocks.Hash(), snapshotdata.RoomNID, newblocksarray).Scan(&newNID)
+					INSERT INTO roomserver_state_snapshots (state_snapshot_hash, room_nid, state_block_nids)
+						VALUES ($1, $2, $3)
+						ON CONFLICT (state_snapshot_hash) DO UPDATE SET room_nid=$2
+						RETURNING state_snapshot_nid
+				`, newblocks.Hash(), snapshotdata.RoomNID, newblocksarray).Scan(&newNID)
 			if err != nil {
 				return fmt.Errorf("tx.QueryRow.Scan (insert new snapshot): %w", err)
 			}
@@ -252,16 +251,49 @@ func UpStateBlocksRefactor(tx *sql.Tx) error {
 	// If we do, this is a problem if Dendrite tries to load the snapshot as it will not exist
 	// in roomserver_state_snapshots
 	var count int64
+
 	if err = tx.QueryRow(`SELECT COUNT(*) FROM roomserver_events WHERE state_snapshot_nid < $1 AND state_snapshot_nid != 0`, maxsnapshotid).Scan(&count); err != nil {
 		return fmt.Errorf("assertion query failed: %s", err)
 	}
 	if count > 0 {
+		var debugEventID, debugRoomID string
+		var debugEventTypeNID, debugStateKeyNID, debugSnapNID, debugDepth int64
+		err = tx.QueryRow(
+			`SELECT event_id, event_type_nid, event_state_key_nid, roomserver_events.state_snapshot_nid, depth, room_id FROM roomserver_events
+				JOIN roomserver_rooms ON roomserver_rooms.room_nid = roomserver_events.room_nid WHERE roomserver_events.state_snapshot_nid < $1 AND roomserver_events.state_snapshot_nid != 0`, maxsnapshotid,
+		).Scan(&debugEventID, &debugEventTypeNID, &debugStateKeyNID, &debugSnapNID, &debugDepth, &debugRoomID)
+		if err != nil {
+			logrus.Errorf("cannot extract debug info: %v", err)
+		} else {
+			logrus.Errorf(
+				"Affected row: event_id=%v room_id=%v type=%v state_key=%v snapshot=%v depth=%v",
+				debugEventID, debugRoomID, debugEventTypeNID, debugStateKeyNID, debugSnapNID, debugDepth,
+			)
+			logrus.Errorf("To fix this manually, run this query first then retry the migration: "+
+				"UPDATE roomserver_events SET state_snapshot_nid=0 WHERE event_id='%v'", debugEventID)
+		}
 		return fmt.Errorf("%d events exist in roomserver_events which have not been converted to a new state_snapshot_nid; this is a bug, please report", count)
 	}
 	if err = tx.QueryRow(`SELECT COUNT(*) FROM roomserver_rooms WHERE state_snapshot_nid < $1 AND state_snapshot_nid != 0`, maxsnapshotid).Scan(&count); err != nil {
 		return fmt.Errorf("assertion query failed: %s", err)
 	}
 	if count > 0 {
+		var debugRoomID string
+		var debugSnapNID, debugLastEventNID int64
+		err = tx.QueryRow(
+			`SELECT room_id, state_snapshot_nid, last_event_sent_nid FROM roomserver_rooms WHERE state_snapshot_nid < $1 AND state_snapshot_nid != 0`, maxsnapshotid,
+		).Scan(&debugRoomID, &debugSnapNID, &debugLastEventNID)
+		if err != nil {
+			logrus.Errorf("cannot extract debug info: %v", err)
+		} else {
+			logrus.Errorf(
+				"Affected row: room_id=%v snapshot=%v last_sent=%v",
+				debugRoomID, debugSnapNID, debugLastEventNID,
+			)
+			logrus.Errorf("To fix this manually, run this query first then retry the migration: "+
+				"UPDATE roomserver_rooms SET state_snapshot_nid=0 WHERE room_id='%v'", debugRoomID)
+			logrus.Errorf("Running this UPDATE will cause the room in question to become unavailable on this server. Leave and re-join the room afterwards.")
+		}
 		return fmt.Errorf("%d rooms exist in roomserver_rooms which have not been converted to a new state_snapshot_nid; this is a bug, please report", count)
 	}