This may also be related to the tests failing on official buildbots as mentioned in [1], although the error messages look different. However both of these issues seem to be a memory corruption problem, as one of the frames in stack trace (the one that fails the tests for me) has ob_refcnt < 0 and ob_type = 0x26000000, (corresponds to tuple if I'm not mistaken) and all sorts of other weird values.

[1] http://bugs.python.org/issue12605#msg166423