I noticed one big flaw. The two guys waiting outside managed to magic a knife into her back while she was inside. Now if they were in there and they wanted to kill the guy, as they did, why wouldn't they finish the job instead of running back outside to wait for him to come out?

One of Hitchcock's very greatest. Very witty and endlessly entertaining, with a perfect combination of humor and thrills.

Maybe the men on the street threw the knife up and into an open window where it landed in the back of the lady? lol