Here are 17 of the best answers, aided visually by the very expressive Phillip Schofield:
1. "When something happens to the main protagonist that is supposed to make you think they're dead, but there's still an hour left in the movie so you know they're alive."
2. "Bombs with a digital countdown clock, apparently put there as a courtesy to anyone trying to disarm it."
3. "The second a pill hits your mouth, it starts working."
4. "The ridiculous middle-aged dad who makes lazy horror movie plots possible. 'Yes, I know human organs just came pouring out of your closet, but give this house a chance, hun!'"
5. "Is cop or military. Sent into action, overwhelming odds. 'I DIDN'T SIGN UP FOR THIS SHIT!'"
6. "Whenever they hack a computer or equipment, it's always quick and easy with just a few key presses."
7. "Creepy, stalkerish obsession and harassment being portrayed as 'romantic.'"
8. "When people drive in movies, the road is always clear in front of them, or they find a prime parking spot."
9. "Main character fights their way through enemy base killing 200 henchpeople in the process. Then, when they reach the villain, they refuse to kill because 'it would make me just as bad as them.'"
10. "Forced romance subplot in a movie that requires no romantic interest at all."
11. "Some scientist or doctor says something deliberately 'smart,' and another character asks for it to be repeated 'in English.'"
12. "Space crew travels to unexplored planet on purpose or by accident. Someone says, 'It has a breathable atmosphere,' and everyone proceeds to get out of the ship in an alien environment completely unprotected. Because the only reason nature ever killed anyone on Earth is because of the motherfucking composition of the atmosphere."
13. "'Wow, you’re pretty tough for a woman.' 'Yeah, I grew up with six brothers.'"
14. "When someone knocks at a door, clearly in urgency. The inside character remains on the sofa. The knock comes again, and again, and again. Then the inside character stands up from the sofa, and even though they are still well outside of hearing range and there is no line of sight to the front door, the knocker stops knocking."
15. And finally, and simply, "American High School."
Note: Answers have been edited for length and/or clarity.