Like sending important emails to the wrong people? lol, just kidding mate.
I've made a couple of errors, the biggest one I suppose was getting 2 paypal accounts locked, with over $500 combined. The horrible thing was, I did nothing wrong?!
I did make a mistake the other day though... I sold a domain to someone with low feedback and they sent a chargeback, so I lost the money and the domain, but then it was only $5, so it's not too bad.
I suppose we all learn from our mistakes