Affects people with software raid, and up to date stable kernels, which I am guessing may be many on this list.
May affect other kernels/filesystems too. You may wish to revert to an earlier kernel.
The current stable kernel contains the bug apparently, do not update yet until it's clear that you are getting the fix
(andnot the bug).
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=785672