Death by Mocking

Don't tease or mock anyone in a horror film. The monster doesn't like it, and will kill you for it. Not that the monster actually likes the people you mock (it will kill them too). It's just that mocking is the monster's thing. You're butting in.

So don't tease people; don't treat anyone like they're crazy for talking about the monster, and don't play any practical jokes. The last one isn't a Deadly Prank, where someone will get hurt. The monster will just kill you later for pulling one.

Note this doesn't count if the people you tease laugh with you. That isn't mocking; it's just having fun.

Compare Death by Sex, Death by Materialism, and Death by Genre Savviness. See also Did You Just Flip Off Cthulhu?

Film

 * In Anacondas: The Hunt for the Blood Orchid, the characters are wading through a river. One guy keeps humming the Jaws theme. A girl keeps telling him to stop (instead of just punching him in the face). He's one of the first victims.
 * Mildly subverted later when another character is taunting a freshly killed anaconda snake, only to be grabbed by another. Unlike the previous victim, the other characters manage to save him from death at the last minute.
 * An aversion occurs in Scream. Two guys pretend to be the killer in school. They don't get killed for this joke, probably because the principal who punished them (and who gets killed later) was even more obnoxious.
 * In a recent Jaws/Piranha knockoff (not Deep Blue Sea), some girls are sneaking into a closed beach. The fence is electric, and one of the girls tests it. It's off, but she acts as though she's getting electrocuted anyway. She gets eaten.
 * Jurassic Park has a similar scene, but the victims laugh at that (well, one of them does), so the joker lives to the end (and even through the second sequel).
 * Lost World: Jurassic Park had a guy who taunted a Compsognathus later get eaten by a pack of them, however.
 * Averted in the second Friday the 13 th movie. Right before Jason really gets started building up his body count, half the characters go into town to go drinking and essentially leave the movie. The practical joker (Ted) goes with them, and survives to the end credits. However, the joker character does indeed go down in parts 1 (Ned), 3 (Shelly), 4 (Teddy), 6 (Cort), 7 (Carrie-esque Rich Bitch Melissa), and 8 (Julius).
 * Carrie was pushed over the edge by a particularly malicious prank that was the culmination of all the nasty teasing, taunting, humiliation and pranking done to her by the girls in school. She proceeds to kill just about everyone at the prom before taking her Roaring Rampage of Revenge elsewhere.
 * Spoofed in Scary Movie, and later in the third film when Brenda pranks Cindy by saying she watched the killer videotape, and then pretending deaths... and as Cindy is out of the room, "Samara" comes out of the television and kills Brenda (specially because Cindy wasn't believing her cries anymore).
 * Steve in 2004 version of Dawn of the Dead. His fault? Bickering with Anna. His punishment? Zombification then bullet through the brain. Riiight...
 * You left out "nearly got the heroes killed by abandoning his post when he was supposed to open the back door and let them in."
 * Nearly happens to Mortimer Brewster in Arsenic and Old Lace, as he's spent a good deal of the first act mocking his older brother Jonathan, who of course later shows up and turns out to be an Ax Crazy Serial Killer who fully intends to make Mortimer his next victim.
 * Averted in Tremors, where Melvin the prankster manages to survive to the end of the film.
 * In Freddy vs. Jason, Kia tries to show she's not afraid of Freddy by mocking him to his face. Jason chops her in half from behind.
 * A pair of ravers meeting Jason for the first time mock him. This does not end well.
 * Anybody who mocks Michael Myers in the Halloween remake doesn't last very long.
 * Night of the Living Dead: "They're coming to get you Barbara . . ."

Live Action TV
"Xander (in a voice like talking to a baby): Who's a wittle fear demon? Giles: Don't taunt it. Xander (getting up, looking scared): Why? Can it hurt me? Giles: No. It's just... tacky, that's all."
 * Buffy the Vampire Slayer lampshaded and then subverted this.