I saw this error recently on some 3.4 builtbots:

http://buildbot.python.org/all/builders/AMD64%20Windows10%203.4/builds/74/steps/test/logs/stdio (failed last 4/5 builds)

[306/390] test_json
Fatal Python error: Cannot recover from stack overflow.

Current thread 0x00000fd8 (most recent call first):
  File "D:\buildarea\3.4.bolen-windows10\build\lib\json\encoder.py", line 250 in iterencode
  File "D:\buildarea\3.4.bolen-windows10\build\lib\json\encoder.py", line 192 in encode
  File "D:\buildarea\3.4.bolen-windows10\build\lib\test\test_json\test_recursion.py", line 96 in test_endless_recursion

http://buildbot.python.org/all/builders/x86%20XP-4%203.4/builds/1224/steps/test/logs/stdio (only failed 1/5 builds, stack trace essentially the same)