well? Ever "done" one? I'll kick off:
1) Shill bidding. This is when you bid on something your mate / family member / someone you know is selling at an auction purely to bump the price up. Fucking "shill" bidding. Yeah I was suspended from eBay once for this.
2) stealing glasses / condiments from the pub. I believe meow left the first Spoons pub crawl with a shit load of condiments. Obviously the only glasses I have in my house are ones I've nicked from the pub.
3) Not handing £20 or whatever you see on the floor into the police. Done this a few times but in my defence there was no one around to claim it. Still doesn't make it okay, perhaps?
These aren't particularly interesting or funny but I bet there are some good stories about these types of things from you lot.