A particularly nasty exploit in the Xbox 360 edition of Borderlands 2 saw players' characters wiped if they happened to play online with someone who used a hack, making the usually-oh-so-friendly Xbox Live a potential deathtrap for your vault hunter. The promised patch to fix it has now been rolled out, so you should be as safe online with random strangers as you'll ever be.
The patch should download automatically next time you go to launch Borderlands 2 while connected to Xbox Live. The only new item in the patch notes is this fix, described: "Added security to prevent data loss tied to malicious violations of the Xbox Live Code of Conduct through an unsupported third-party program."
The hack was initially used by players to edit save games to enable Hardcore mode, but had the nasty knock-on effect of breaking the saves of people who played with them. As you might expect, some saw the potential for naughty giggles and went gallivanting around, killing characters.
PC and PlayStation 3 players were safe from the hack all along, so no worries there.