I hate people who run around and just post things like "wrong forum". Usually, if someone posts a topic in the wrong forum, I check their join date. If it's relatively new, then I'll let them know. If they've been here a while I either ignore the thread, or just reply to it normally.
I don't bother letting older members know it's a wrong forum, because it doesn't make the thread move to its rightful place, they should already know what goes where(in most cases) and I can always count on some pompous forum cop to do it for me.
There's always exceptions, where it's tough to decide where to post it, and that's acceptable, but when someone from 2005 posts a Halo 3 weapons idea thread in the Underground, I just ignore it.
In my mind, there's a difference between a helpful member and a forum cop, and I think the mods think the same way. A helpful member is someone who points members in the right direction with threads, or letting someone know that the topic is a volatile one, and to be a bit careful.
A forum cop is one who runs around saying wrong forum, but doesn't let the member know what forum the thread belongs in, posts crap like "3,2,1, locked", flaming members for making stupid topics instead of just PMing a mod, and generally acts like a mod.
[Edited on 10/9/2006]