My brothers deleted my Gran Turismo 1 file. They thought that they could delete a single block from my save file in the memory card....
Extra: The worst game file loss I've caused.
A friend lend me his Dreamcast for a few days. There was a power outage when Soul Calibur was autosaving (he had it on and I didn't touch the options) and the data was corrupted. I spend an entire day unlocking everything again, but the records were lost.