WARNING: POSSIBLE SPOILERS FOR HIGH TENSION, I AM LEGEND AND FIGHT CLUB
From this discussion in Flickr Name That Film:
I Am Legend had a disappointing ending, although I still liked it. It would've gone from good to great if they'd chosen an ending closer to the book. The Last Man on Earth had the closest to the original ending, unfortunately the mutants there looked like people with a bad hangover.
The ending of High Tension totally ruined it for me. If you're going to do the "unreliable narrator" thing in a movie, you have to drop hints or set it up in some way. I felt the same about Fight Club, although it's a much better movie. Just no set up at all for the twist. I haven't read the book, I assume the author did a better job.
Successful movies with an unreliable narrator:
Kind Hearts and Coronets
The Cabinet of Dr. Caligari
The Blade (1995)
Now playing: Big Audio Dynamite - E=MC2