Found in the stdlib test suite while testing the fix for bpo-6721: When subprocess is called with a preexec_fn and os.register_at_fork has been used and the fork() system call fails, the Python error state is set before the "after fork in parent process" function calls that were registered were called.  So they'd call back into Python code with a pending unreturned exception set.

boom.  SystemError (or assertion failure in pydebug builds).