I accidentally entered to admin.php?tools/run-deferred on beta 5 and the page was continuously loading something while it said "Processing..." It this url is trying to rebuild something that has finished, I suppose it should shows a message saying that the execution is not allowed, right?
I can't reproduce this and I'm not sure how it would happen unless it actually doing something. If there's nothing to do, it will redirect you out. Maybe you were just being redirected back to it? I suppose that could be it, though I'm not sure how it could be triggered off hand.