My favourite one is that at some point there was a lot of piracy happening in a forum. Someone would post "Hey, I'm looking for <Game X>, thanks", and someone would reply "Here's a link to the zip: <link>". Sometimes people would reply "the link is broken", or "thanks", or "the zip has a virus".
After fuming for a couple of days, I discovered that you could post to this forum without needing to register, and that you could post via a simple HTTP form post (this was before REST APIs). I figured that if the website couldn't be taken down (guess which country hosted it...), at least I could make it less useful!
So I wrote a bot that would periodically scan for posts asking for zips, and post replies with bit.ly urls that went nowhere. It also scanned for replies to posts that contained links, and randomly replied "the link is broken", or "thanks", or "the zip has a virus", all using a simple text generator with phrase templates, an username generator, and a game name generator.
What I didn't anticipate, but was obvious in retrospect, was that the bot started posting fake replies to its own fake posts. So for a few days, my bot was essentially running the forum by itself!
I eventually fixed this, as hilarious as it was. To get around IP blocking and rate limits (which I'm unsure I ever actually found), I distributed this bot to a handful of fellow indies whose games were being pirated in the same place as well.
I don't remember how or why I stopped using this, or whether it ever made any difference to our bottom line, but it was a silly source of entertainment for a few weeks :)