In October 2011, Mt Gox lost over 2609 bitcoins by accidentally creating transactions that can never be redeemed. (At the time, 2609 bitcoins were worth about $8000, but they would be worth almost $1.5 million dollars now.) The problem was discussed on the Bitcoin forum, but I think it's worth taking a closer look at it in light of Mt Gox's recent massive bitcoin loss and bankruptcy. This problem reveals how easy it is to lose bitcoins through software errors. It also reveals that Mt Gox has made careless and expensive programming errors before.