Ok, I'll bite.
I agree with the above regarding people posting "for sale" items in the forums.
Another one that pops up from time to time is people failing to check the date on a post or thread before replying.
Occasionally you will see a post dredged up from the depths by a new guy trying to be helpful, saying they've got what the OP needs, telling them where they can get it or how to do it, without realising that the OP posted the request/question in April 2004 and hasn't been back since May 2004.
Resurrecting a dead thread can often suck others into replying to someone who will never see the reply. I will, however, concede that it may help someone else who is looking for the same information.
So please, check the date on the last post before replying.