In my opinion, the worst anti-pattern both for its nefarious effect on the community and because of the difficulty in confronting it is the "Free Software is all about absolute Freedom" way of thinking. This kind of thinking breeds a kind of anarchistic community where a substantial proportion of members affirm a total right to freedom of speech, freedom of expression, and other extreme libertarian principles.
The result is a community where no indiscretion can easily be corrected, because any attempt to do so is perceived as censorship - a limit to the absolute freedom of speech. The more people you have in your community who believe this and express it vocally, the more your community will attract. And people who believe in absolute freedom of speech are also the people most likely to exercise it - they speak first, loudly, often and longest.
Some notable examples have come up around moderating mailing list behaviour and wiki pages syndicated to a Planet at GNOME.
The cure for this is, in my opinion, usually amputation, which makes it very difficult to cure. A critical mass of respected community members, or a BDFL, must eject the free-speech-loving element for the good of the community, or very clearly enforce rules on behaviour until they leave or quieten down. Because the cure is so radical, it is often extremely difficult to solve this issue.