I can think of a few:
One time I registered on a different gaming forum 12 years ago with an inappropriate name. As soon as I clicked register, it did so successfully. It then took me back to the main page where I had a message in my inbox saying that I have been banned because of my screen name. And this was INSTANTANEOUS, mind you. So it’s not like a mod immediately noticed and gave me a ban. It was automated. Well geez, if it can detect profanity on the fly, why didn’t the registration form just come back win an error message (like when you enter an invalid email, type password incorrectly in 2nd box, etc)? And at the time, I had only my one email address that my dad set up. I knew nothing about setting up alternate email addresses myself. So I was extremely pissed.
I was using profanity in a post and doing what most people do; replacing the entire word with asterisks. But the forum I posted on has an auto-censoring function. And doing anything at all to bypass it, including the substitution of the word fudge, or replacement of the ENTIRE word (not just a letter or two) with asterisks gets you a warning. No other forum that I have ever posted on functions like that.
I was posting on an unofficial starcraft forum, asking if there was a widescreen mod for the game (similar to widescreen mods in other isometric-view games like Baldur’s Gate). There was one somewhere on the net, but it involves altering some of the core files and, arguably, gives players an unfair advantage in multiplayer. So I got kicked because what I was asking for had constituted asking for multiplayer hacks/cheats.