Some of mine: (keep in mind I'm not going to count "that movie was terrible!" because that's a completely different topic)
-Any movie I walked out before it was over, for obvious reasons. Felt like I wasted my money.
-Dracula (1931, Spanish version): Saw this at a cemetery. That was the cool part. What wasn't cool was this cemetery didn't have modern bathrooms, or even kept up very well. The one stall was missing the door and there were worms crawling on the wall. I can still picture it. Worst bathroom ever.
-Goonies, The: A movies-in-the-park thing. Because it was outdoors, lots of people around me were smoking. Not a fan.
-It's a Beautiful Day in the Neighborhood: Wasn't able to finish it, not because of the movie's quality, but because I thought I was having a heart attack. An urgent care visit later and it turned out to simply be heartburn. Getting older: It's fun!
-Legend of Boggy Creek, The: Whatever video file they were using for this movie was corrupted or poorly encoded or something, because the audio played very distorted. It sounded like all the characters were underwater.
-My Little Pony The Movie: Due to the slow ticket lines (this was right before movie theaters allowed you to buy tickets at a touch screen), I missed the Hanazuki short before it, which irritated me, especially since I was there a good 15 minutes before start time.
-Pirates of the Caribbean: Not for the movie itself, which I enjoyed, but I think the air conditioner was broke, so it got pretty hot in there. [side note: This was also a memorable screening because one of the audience members was enjoying some of the jokes a little TOO much]
-Rock and Roll High School: Had a headache during it. Not great for a movie that contains a lot of rock music.
-Simpsons Movie, The: Probably had the most "audience members reciting lines from the movie" than any movie I've ever watched. It got old, fast.
-Stuart Little: Why was a 16-year old going to this movie, and with a friend no less? I definitely felt too old to be there.
-Toy Story 1: Because I never got a chance to see it on the big screen. The projection machine broke or something. A theater employee came out to deliver the bad news that this screening was cancelled but was nice enough to give us vouchers to another movie of our choice. I forget what we ended up using it on, but it wasn't used the same day so we went home having watched nothing. We lose, good day sir!
-Wish: The Beyonce concert movie was playing next door and it was LOOOOOUUUUUD. Made it hard to concentrate on the movie I was actually there to see. That might qualify as the worst ever, now that I think of it.
I forget which movie it was, but there was one which was affected by a power outage so I never got to see how it ended. All I remember is it was a vintage movie.