Star Trek Online. And yet for some reason, after the right barrage of questions about how you loot things in Final Fantasy XI, where you can get travel powers in City of Heroes, or how to train pistols in Fallen Earth... well, eventually you just snap. You give someone advice that isn't just bad -- it's malicious.
Almost every veteran has done it at least once, ranging from assuring a player that the dragon is totally soloable to explaining that black dye is cheap and unwanted. So what's the worst thing you've ever pulled over on a new player? Was it something that got the character killed, or even worse, something that forced the player to throw away what could have been a long-term benefit? And in the unlikely event that you don't have a cruel sense of humor, have you ever been on the receiving end?