500 buy on 140, then 500 sell back to 136 in 5 seconds.
00:37:43 > Current trend: -3.99999 USD, Volume:536.6481 BTC
Profit: -36.60 BTC. Ouch. Make sure your buy and sell orders don't overlap, guys...
(This will happen if your orderbook has a limit ask for some low price, and you later enter a bid order for some higher price.)
PS: Wadn't me... This time. Also: Mt.Gox, fix this shit!
It happened to me few days ago, finished with 3$ higher average entry. I'd have at least 30-40% more coins if it was not due to mistakes like this one, buying when wanting to sell, stupid currency exchange mistakes etc.. one second mind block and you're down 2-3%.