We had a small group of friends and/or religious zealots (Discordians) from all over the world, and we'd pick a medium-smallish webforum on a random topic and we'd go on safari. Dress up our avatars with little pith-helmets, register an account and uh, explore the local wildlife.
We didn't really try to disrupt or actively troll anything, just have a bit of fun and, well, basically we'd roleplay stereotypical Bri'ish explorers and I dunno it was hilarious :-)
Reactions varied widely. Sometimes moderators just got spooked because five new accounts registered all at once and they were acting silly and it looked like a troll invasion (but we were always nice and polite and we offered them tea) depending on their sense of humour, sense of adventure, sometimes we'd get banned within minutes, otherwise we'd hang around for a day or two, check out some discussions, actually being helpful if we could, just play in the "general discussion" sub or photoshop some pterodactyls on their memes. The better forums really got in on the joke, great times were had, and occasionally some of their forum members joined our little band. If we didn't get banned, we'd try to leave a link to the next place we would explore.
- I once came across a nomadic TV watching club - they'd decide to watch a classic TV series, and then go to the board for that series to hold weekly discussions (you go in one week and see them discussing the first episode, then go in the next week and see them discussing the second, etc.). When the series was finished, they'd announce the next series they would watch and migrate to that board.
- At one point the board for Kingdom of Heaven was a bunch of history buffs discussion history (and occasionally mentioning the movie).
- Coming across threads that had been going on for over a decade was always interesting. Someone made a comment, a year or two later (long after they've left) someone else comes along and responds to them, a couple years later someone else responds to that, etc.
- The discussion of movie finances could be really informative, and definitely gave me a much deeper understanding of how things work and what it means for a movie to be profitable (basically, it's really complicated).
And of course, the boards were a great place for things like recommendations, up to date news, and learning about things you missed in the film.