My pet peeve: abusing the fact that you're writing in a fantasy/sci-fi setting. This includes pulling never-before seen magic/science out of a hat and using it to get the characters out of the impossible corner you've written them into.
For an example, see J. K. Rowling or for a writer who uses magic appropriately, see George R. R. Martin.

RE: Anoymous - Hey, it's the Guardian. What did you expect?

RE: Xopher - they tried that and it didn't work.