I was just merging 2to3 work (and you'll have to do svnmerge.py merge in
Lib/lib2to3 for this to work) into py3k and was running the tests when
this rather alarming thing happended:

$ ./python.exe Lib/test/regrtest.py test_lib2to3
...
test test_lib2to3 crashed -- <class 'UnboundLocalError'>: local variable
'result' referenced before assignment
Traceback (most recent call last):
  File "Lib/test/regrtest.py", line 601, in runtest_inner
Bus error

Thinking this had something to do with r64281, I reverted it and got:
test test_lib2to3 crashed -- <class 'UnboundLocalError'>: local variable
'result' referenced before assignment
Fatal Python error: GC object already tracked
Abort trap

This probably isn't a lib2to3 problem, but that's how I got it.