In case you can't watch that video, the lady vampire is attacked by the man's cat collection. Cats fly at her from all directions and drive her away from her intended victim, or more accurately, the actress is pelted with unmoving cat puppets like she's in some PETA-unapproved version of dodgeball, and said kitty-puppets then pretend to bite her in an orgy of unintended puppet-hilarity.
Why It Ruins Everything:
When I first saw this scene and experienced the audience around me guffawing in hearty laughter, I thought it was because the cat-puppet effects were so bad. Because let's face it: It's a Swedish horror movie, they weren't working with a Michael Bay budget here. But now I think that there's more to the awfulness of this scene than just the "bad puppet" aspect. Even if the filmmakers had spent $56 million and produced a photorealistic cats-attacking-vampire masterpiece, the scene still would have been ridiculous, because cats on screen just can't be that horrifying.
Look at this guy's adorable little head!
"Cat yowls off screen after a character throws something" is a comedy staple for a reason, and that reason is not that animals in pain are hilarious. Cats just have a tendency to be inherently funny. I think it's the gulf between their dignity and their real-life helplessness. A small dog falling off a chair is not funny. A cat falling off a chair is funny, because a cat will fall off that chair with an air of quiet, self-assured dignity, and then look at you afterward like it meant to do that.