Borderlands 2 introduced a new system by which completing challenges (25 shotgun kills, 100 heads blown off, etc.) unlocks "badass points" to spend on stat upgrades that carry between characters. Understandably, it's quite useful.
But now a bug that appears to be related to save file corruption is ripping through the Borderlands 2 community, erasing badass points but leaving the challenges as they are, so that those points can't even be re-earned. Needless to say, this is not good.
Gearbox community manager Chris Faylor has stated on the forums that Gearbox is looking into the issue, but in the meantime other forum users have come up with some possible preventative measures (on Xbox 360 and PS3). The Xbox 360 instructions just say to wait 30 seconds once you quit to the main menu before you turn off your system. On the PS3 it's a little more complicated.
No word yet on what PC gamers should do, or whether players who've lost their badass points will ever see them again. It sucks to lose progress, and hopefully Gearbox will address this issue soon.