If you haven't encountered Queryfail (the brainchild of literary agent Colleen Lindsay, it's an initiative in which agents and editors post to Twitter about How Not To Query), it's definitely worth checking out. Jane Smith gives a good summary on her blog, including some of the negative reactions to the project.
Such discussion can be bruising, especially if it's your query being Queryfailed. But in my opinion, the information provided--direct from the source--is extraordinary helpful, and writers should make an effort to take heed. Also, given the incredibly unrealistic expectations many writers have about how agents should respond to queries, and the horrible things that so many writers routinely say about agents, in public and in private, distress over the supposed negativity of agents' Queryfail comments is a bit ironic.
So, joining in the spirit of the thing, here's the ultimate Queryfail: querying someone who is not an agent or an editor. Whose email address, moreover, has "beware" in it. And attaching your entire manuscript to your query.
Yes, I just got that very query. This may seem strange or absurd, but I get queried on a semi-regular basis, despite the lack of anything at all, anywhere on the Internet, to suggest that I am an agent or a publisher. Believe it or not, I have regulars: the guy who sends me another query every now and then in hopes I've changed my mind and become an agent, the guy who snail mails me his horrifying screenplays every six months or so. There was also the guy who, angry at my refusal to critique his manuscript, put me on his spam/chain letter list. I had to report him to his ISP for abuse.
I used to respond individually, but I no longer do, since I feel very strongly that writers need to take responsibility for doing proper research. Instead, I've added a disclaimer to Writer Beware's contact info, and also to my autoresponder. By then it's too late, of course, but at least the queryer will know why their query failed.