Oooooh I hate it when I can't remember the name of a great movie, which I can't here.
But - I can say that this one IS on TCM, with the ending where the director asks not to reveal the ending. Not only that but TCM has also played the trailer for it where they also ask folks not to reveal the ending after seeing it. I am drawing a complete blank right now because I was just reading the Double Indemnity thread and that keeps popping into my head but that's not it.
I'm new to this site (although I've been an avid TCM fan for years) there may be a way to find it here. Not sure if you can search for the trailers they play in between the features, but that might be a short list for you to look for.
Good luck, and if I think of it, I'll happily get an answer up here.