Giving code samples which don't actually reflect your failing code
Giving irrelevant or overly long code samples
Saying "it doesn't work" but not in what way
Saying "it throws an exception" but not stating the exception type or message
Writing contradictory questions in the body and title
Writing one question, and then commenting when that question is answered, "Oh, I meant [some other question]"
Not responding to requests for further information
Giving arbitrary restrictions without reasons for them (when those reasons may very well rule out other options too)
Failing to format code (even when it's blatantly obvious that it looks horrible)
However, I think just a list of things to avoid isn't the right approach. I once blogged on how to answer technical questions helpfully - I'd be happy to write a similar post on asking questions usefully, and for that to used as part of a FAQ entry, if that would help.
Not explaining what they actually want to achieve. Zero background information just slamming in a code sample that isn't working for them.
If people would explain what they want to achieve besides what is not working for them the answer could be of better quality, because maybe what they're doing is a (completely) wrong approach besides bad coding.