Legion (2010)
It's the only one ever. Me and 4-5 college friends went to see it. We all collectively decided to leave at the same time. Can't remember the specific moment but we all looked at each other and silently agreed to leave.
I can't say I've had that feeling much. I usually don't go to a movie unless I really want to see it and I'm quite positive it will be worth my time. Usually pretty easy to tell from previews.