" I was watching, like many of you, a gigantic sell order burning through the bids. Mt Gox doesn't execute trades very quickly, so we were watching this huge order slowly eat up every buy order on the books. The price started at around $17.50, and within minutes was below $10. At this point, I realized this wasn't merely a large seller willing to accept some losses. This was someone attempting to crash the market by selling a huge percentage of the market's total bitcoins at once.

I had around $3000 USD in my Mt Gox account, from earlier sales I'd made. I looked at the market stats, and realized that there were tons of orders to buy BTC at $0.01 that would likely eat up any remaining bitcoins this seller had on the order. I figured if I put a buy order in for $0.0101, my order would execute first and I could buy a huge amount of bitcoins from this seller before it hit the bottom. The only problem was that Mt Gox was running slower than molasses at the time, and everyone was saying that it wasn't accepting trades. I had to try several times, but eventually I got a buy order in, offering to buy as many bitcoins as I could for $0.0101.

The site stopped responding completely for a while, probably from so many people hitting refresh to see what was going on. When I got back in, I saw in my account:

06/19/11 17:51 Bought BTC 259684.77 for 0.0101

I had just purchased over 250,000 bitcoins for $2613. At the trading price immediately before this large sell order happened, that number would have been worth nearly $5 million. After I regained my breath, I tried to figure out what to do. I wasn't sure what was really going on. Over the past few days there had been a lot of talk and complaints about Mt Gox's security. Lists of Mt Gox usernames, email addresses and encrypted passwords were being traded around, which was an obvious sign that security at Mt Gox had been breached in some way. If there was an attacker in the system, perhaps he was able to log into my account now and force my account to execute some other crazy trade. "

Source: https://bitcointalk.org/index.php?topic=20207.0