Basically, a user downloads your software, and finds a bug; but, instead of actually telling you about it, they angrily "go public" and rant somewhere. If you're really unlucky, they'll choose a place with enough pageviews that your project's reputation is quickly tarred, and others wonder why they should even bother to download.
What's really insulting about this behavior, is that these are people who have sent no E-mail, filed no bug reports, and donated no money, code, time, or anything else. The bugs can be obscure, and even hard to reproduce. These people feel entitled to shred your years of work: they spend a few minutes clicking on something, and head straight for the forums.
How do you deal with this?
I can tell you as a maintainer that it hurts, but I still try to suck it up, and I respond to even destructive criticism with action. If I can figure out what problem the person is referring to, I fix it, even if I'll never be able to tell them that it's fixed (and they probably wouldn't care if I did).
But sometimes I think people need to be educated again. Clearly anonymous posters feel invincible in a way that wouldn't translate to real life. Imagine coming to my house, receiving a wonderful free meal, and then throwing it in my face while screaming how horrible it is because it's missing a dash of salt?