Can't really think of a "Hmmmmm" moment, best I've got is a bunch of "WTF?" moments
Like the end of Safety in Numbers (2006).
So it's a slasher movie set on a tropical island. By the end, we see our two survivors on a yacht headed for safety, and the killer, badly injured, walking on the beach. So obviously they've escaped him, right? But there's a second killer on the yacht. At absolutely no point in the movie did they ever hint that there was a second killer, there was no explanation whatsoever; it was just some stupid twist they tacked on at the end that made no sense at all. I went over to IMDb thinking somebody on the message boards might have an explanation, but no - everyone is just as confused as me.