A malicious attack on Borderlands 2 players is rendering save files irretrievably broken on Xbox 360, according to Gearbox. PS3 players are unaffected at present, although unofficial community forums are reporting the issue is also affecting PC players. The attack is being dubbed the 'Graveyard Virus' because it means when a character dies in the game, they never come back. Ever.
The virus works by silently switching the game's difficulty setting to Hardcore mode, which means you only get one life. Fail to get a Second Wind during 'Fight for your Life' and your character will die, dumping you back to the title screen. Gearbox advises immediately selecting 'Continue' if this happens, and always ending a game by pausing while alive and selecting 'Save and Quit'. Doing anything else will likely lose your character forever.
The virus is being spread through duplicated and shared items, so the advice being given by mods on Borderlands 2 forums is to play only in private online matches with people you know (although they'll have to still be 'clean' to avoid passing anything on).
A patch is already in the approval stage so the problem will be fixed soon, but in the mean-time it's much better to be safe than sorry. We can't imagine losing our level 41 Commando. Or that corrosive handgun we got from Tiny Tina. Best tea party ever.