"This Is Spinal Tap" is an absolute classic! I saw it when it originally came out in theatres, and it was a riot. But then I saw it again when it came out on video, and I watched it at my mother's. She was one of those that was convinced that it was a real documentary about a real band. I haven't let her in on the joke yet (and that was 15 years ago).
Anyway, back to the topic of the thread. How about "Hercules in New York" for a real mood-killer. This was Arnold Schwarzenegger's first movie, and he is titled as "Arnold Strong." The director thought his accent was so thick that it would never be understood by American audiences, so they did a voice-over for all his lines. Now that we all know what he actually sounds like, this makes the whole thing hilarious - until you realize just how bad the film actually is, and then it is just sad.
-dave