Is it possible that PyEval_GetFrame returns incomplete frames. When this happens, Python segfaults in release mode or crashes in debug mode because of the check for incomplete frames in _PyFrame_GetFrameObject. This is reproducible in many ways involving C extensions, the easier one may be by installing a custom memory allocator for all the domains and calling PyEval_GetFrame from there. As the allocator can be called at random points of the eval loop (for example here) is perfectly possible that a legitimate call to PyEval_GetFrame returns an incomplete frame.