Comment by Xdes

MtGox wrote their own Bitcoin wallet software.

MtGox had a bug in the wallet software that caused it to lose track of outgoing Bitcoin transactions.

People exploited that bug to take out more Bitcoin than they had in their MtGox account.

MtGox shut down Bitcoin withdrawals to fix the bug.

MtGox has not fixed the bug three weeks later and a lot of their customers are anxious.