556
News reports such as this one indicate that the above number may have arisen as a programming bug.
A man in the United States popped out to his local petrol station to buy a pack of cigarettes - only to find his card charged $23,148,855,308,184,500.
That is $23 quadrillion (£14 quadrillion) - many times the US national debt.*
In hex it's $523DC2E199EBB4 which doesn't appear terribly interesting at first sight.
Anyone have any thoughts about what programming error would have caused this?