A hyper-link that is claimed to be something that it isn't.
Say someone posts a link, and that link, the word "kangaroos", but the link actually takes you to a site about penguins. It's just link being Rick Rolled, except in a link, and can be much worse.
Forum post
Haganashi93845whatever at 21:32:08
Take a look at these cute puppies (link in the word puppies takes you to 2 girs 1 cup flash video)
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shimohiatu9847389whatever at 21:55:42
EEEEEEEEEEEW WHY DID YOU MAKE ME LOOK AT THAT!?!?!? ASS HOLE.
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IAMTECHNOVIKING at 23:23:23
Technoviking is please with your Trapdoor Link.