It's unlikely anyone's using this in production, but......

[Phoronix] EXT4 Data Corruption Bug Hits Stable Linux Kernels

As far as how you get hit by this EXT4 bug, Ted (Tso) says, "Well, the problem won't show up if the journal has wrapped. So it will only show up if the system has been rebooted twice in fairly quick succession. A full conventional distro install probably wouldn't have triggered a bug... although someone who habitually reboots their laptop instead of using suspend/resume or hiberbate, or someone who is trying to bisect the kernel looking for some other bug could easily trip over this --- which I guess is how you got hit by it."
Gist: If you're running Kernel 3.4.*, 3.5.* or 3.6.* with an EXT4 filesystem and reboot twice in quick succession, you can lose data.

Patches are being worked on now.