1. Friday the 13th
The Plot: Teen counselors had sex when they should've been watching on kids swimming. Child "drowns." Child's mother goes berserk and goes on a killing spree. Child ends up coming back years later as an unstoppable juggernaut who really doesn't like teenagers who have sex.
The Solution: Don't have sex on the job. Those teen counselors are indirectly responsible for the deaths of over a hundred people. If they were doing their job as counselors, they could've saved Jason. If Jason never drowned, then he'd never become the iconic hockey-masked killer, and literally over a hundred people would be alive. Not only that, but Jason would never be able to help restore Freddy Krueger, which put hundreds of more lives at risk. Even ignoring that one itty bitty mistake, if the teenagers just chose not to have sex in a camp ground, then Jason wouldn't have killed them. It is a law in the Friday the 13th universe: if you have sex, you will die. If the teens that go to Camp Crystal Lake just understood there was a time and place for everything, then 80% of them wouldn't die. Jason's anti-sex senses wouldn't have gone off, and the teens would be able to walk away. Unless you're a minority, in which case...Jason's a little bit racist.