I had that very problem at North Weald in the C150. It had a five or six point harness and part of it was trapped in the door. Heard a banging during the run-up and opened the door to look outside and there it was. I don't know about every one else, but I tend not to use the shoulder straps unless it's an all in one seatbelt or I intend to fly upside down.
One thing you might want to think about though is whether you heard it banging at all before you took-off. It's easy to run through the pre-flight stuff in a semi-auto fashion and sort of notice something but ignore it, and assume that all is okay like usual, when really it is not. That's perhaps something worth considering.