Dragonball Online has a captcha that is quite fun.
Every so often a box pops up with dragonballs floating around in it and you have to pick the ones that match the captcha.
If you get it correct you get a buff. If you get it incorrect you get a warning. (Up to 3 warnings I think before you are investigated.)

The main problem I have with said system is that people who are simply AFK and like to be in game (like me) are hit with this system and are not there to respond so a lot of legit people get caught up in the mess.

I suppose one way to circumvent that would be to have a player activity check before launching the captcha (if thats possible.)