Don know if this is old news, as I am rather fresh in the game. This happened to me, i was lucky as it was with profit.
I was in the buy/sell page on Bitstamp and had several tabs open in Firefox, switching back and forwards. Jumping back to Bistamp, then Firefox told me to resend information to page. I pressed OK, and suddenly it put all my btc on instant sell order. Emptying my wallet....
This was not my intention. Maybe this was the same that happened to the person on btc-e some time ago ? Anyways, just a heads up.