Xbox 360 players of Borderlands 2 need to be extra careful at the moment as some players are using an exploit that could see your save game and character deleted forever! in what Gearbox are now classing as "actual sabotage".
It seem that users of the game have been using this exploit to hack their save game files to allow access to a hidden hard core mode, not unlike that found in the game Diablo 3. The mode was left in the games code by the developers, but is only accessible to those who tamper with the code of their save game files using modding software.
Gearbox has already patched the issue, but due to the way updates work on the Xbox 360 this update is having to go through Microsoft's testing and certification process and is still pending it's official release. For the meantime it's recommended that you only play Borderlands 2 online with people you know and trust, most importantly with people you know aren't using this mod.
The exploit has been nicknamed the "Graveyard", as when players die, their characters are wiped forever, but if you're in a game with someone using this exploit, you run the same risk of permanent death.
This issue only effects the Xbox 360 for now, but that's not to say that you shouldn't be aware of the issue and it's dangers on both Playstation 3 and PC. The benefit of PS3 and PC is you can backup your save game files, so I would suggest you make a copy just to be safe.