I think you're thinking about this incident? https://en.bitcoin.it/wiki/Value_overflow_incident https://en.bitcoin.it/wiki/Value_overflow_incident In that case

I think you're thinking about this incident? https://en.bitcoin.it/wiki/Value_overflow_incident https://en.bitcoin.it/wiki/Value_overflow_incident In that case, ~9 hours of transaction history were reversed because of a transaction that resulted in an overflow in the main (and I think only at the time) Bitcoin client allowing the creation of unlimited Bitcoin.