Why are there rectangle shaped glitches when using motion blur problem in Cycles? I'm creating an animation for a client and am getting rectangle shaped glitches in my renders when motion blur is on.

I've tried fiddling with various settings to no avail.
Any suggestions ( not using motion blur is not an answer BTW! )
This is a bug, not a settings issue or user error. Go ahead and report it in the bugtracker. (Though it's probably been fixed since 2.75)