Grease
The waitress must have psychokinetic powers. Right before the song "Beauty School Dropout" she hits the lights off with her hip. Except, she doesn't because the light switch is all the way up the wall. She definitely "misses" the switch, but the lights still turn off. They acted like we wouldn't have noticed!
Harry Potter and the Deathly Hallows
This was actually a big deal in the Harry Potter fandom. Even though the author of the novels had a good amount of say in these movies, one big flaw remained: Harry Potter's eye color compared to his mother's. In the books, she had green eyes. In The Deathly Hallows, we see that his mother has brown eyes. What's up with that? Die-hard Potter fans weren't happy about this.