In the film "Falling Down", Michael Douglas rails against various injustices he observes, from a corner store over-charging for soda and a burger that doesn't match up to the advertisement to a suburban gang taking ownership of a neighbourhood or a homophobic surplus store owner.
At the end of the movie, surrounded by police officers, he asks "who are all these people?" and Robert Duvall answers "These are the good guys".
Douglas thinks for a second and says "I'm the bad guy? How did that happen?"
A community member with a pet hate can be an annoying thing - yes, maybe the newcomers to the list are top-posting, but it is not helpful to say so every single time you see it. Often the person with the pet hate overestimates the ill caused by the infraction, or the significance that others in the community assign to it. By being overly zealous, this community member is driving away newcomers, and getting on the wrong side of established community members.
Typically, when confronted with the issue, the reaction is that of Michael Douglas: "What do you mean I'm that bad guy? I'm just keeping the list safe from top-posters!"
Richard Stallman has been criticised for his insistence that systems based on the Linux kernel be called GNU/Linux. He may have a point, but his repeated insistence on the use of GNU/Linux has alienated him with a large chunk of the free software community. He also insists, slightly less controversially, that people not refer to "Intellectual Property" (because this is a conflation of several very different things), and insists that he is a supporter of Free Software, and not Open Source.
Bradley Kuhn also comes to mind, for his repeated attempts to get Canonical executives to change their stated position on copyright assignment.
Other examples which come to mind are people who regularly remind everyone that they should not top-post on mailing lists, that there should be no trailing spaces on source code, etc. Anything which is, in itself, a good practice, but which most people would overlook.
The cure is often a simple private word from a number of core maintainers off-list. If the person really did not understand that they were being disruptive, then they may stop. However, be aware that there is a distinct possibility that they will go to the opposite extreme of the spectrum, and give out to anyone else who asks a newcomer not to top-post. What's sauce for the goose is sauce for the gander.
If the behaviour being pointed out is by concensus not a big deal, then that warrants documenting in your community norms.
If, on the other hand, people agree that the behaviour is not to be condoned, then figure out a way to spread the policing load around. No-one likes to be seen as the guy who is always complaining.
For the polar opposite (when no-one jumps on pet hates) see anarchy.