MtGox charged me a $45,300,000.0000 for selling 10 BTC today.
I made the sale just as they came up for business, or at least I thought they were up for business. My guess is they they were still running tests but the system allowed me to make the trade. I was very surprised to see a $-45,299,848.52300 in my account.
I submitted a ticket to the service department, and next time I checked the account, 4 hours later, all was back to normal. The sale is still valid and no fee is showing at all for the transaction.