[Jackso]

This has gotta be up there...

http://www.youtube.com/watch?v=qsC-mlpTEOs

Any other appropriate ones?

[The Stig]

All the ones which make a rubbish film look really good / funny. Then when you actually go and watch it, you realise all the good bits in the trailers were the only good bits of the film!

[Phalanges]

United 93 has a pretty bad trailer, sells it as some kind of action film when it's really not. A 30-second clip of the planes going into the Twin Towers followed by the shocked silence of everyone would have been much better.

[Annora]

Hollywood is really good at making trailers. They usually make the film look a lot better than it actually is.

[Ape Gone Insane]

Human Centipede