I tried deleting another bug and it worked without problems. I believe the difference is that the first one had bugnotes.

In core/bugnote_api.php, function bugnote_delete_all():

Delete the corresponding bugnote texts

$query = "DELETE
FROM $t_bugnote_text_table
WHERE id='$t_bugnote_text_id'";
-$result = db_query( $query );
+db_query( $query );

This should fix the problem.

Fixed in CVS. Will be included in 0.18.0a4.