Upon further consideration, I don't think this will address the issue. If the RuntimeError was not being raised, this failure would be consistent rather than intermittent. 

I think I have have gotten a bit mixed up, even if the return value of PyErr_Format is NULL, it would not implicitly return NULL upon being called and exit the function early (like a macro could) . I'm not experienced with programming in C, I only started learning it more recently (Python, Java, and C# have been my primary languages) when I started contributing to CPython in June.

Instead, I suspect this is likely a concurrency issue, where multiple threads are trying to access the same sub-interpreter during ``interp_destroy()``. The solution will likely involve finding the correct place for a lock. I'll continue to work on this and see if I can find a solution.