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Direct from MyBitcoin's Terms of Service agreement. You agreed to it when you signed up, so too bad.
8.1 While MYBITCOIN LLC will make every effort to ensure the MyBitcoin System is accessible at all times, MYBITCOIN LLC makes no representation that User will always be able to access the MyBitcoin System or the User's User Account and User agrees that MYBITCOIN LLC will not be held liable for any loss or damage, whether direct or indirect, resulting from the use, operation or performance of the MyBitcoin System and/or the Bitcoin Network other than as a result of the deliberate or negligent acts or omissions of MYBITCOIN LLC.
".
I would think the serious flaw in the programming of their cart and money flow system would fit under that exception...