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FWIW, rev58032 introduced this:
 tstate = PyThreadState_GET();
 if (++tstate->recursion_depth > Py_GetRecursionLimit()) {
 --tstate->recursion_depth;
 PyErr_SetObject(PyExc_RuntimeError, PyExc_RecursionErrorInst);
 return;
 }
above this line: 
 PyErr_NormalizeException(exc, val, tb);
Contrary to (what I understand from) Amaury's analysis, ISTM that the
call to PyErr_SetObject is the problem, as after the recursion limit is
hit PyErr_NormalizeException isn't called again.
Commenting off the PyErr_SetObject line suppresses the "undetected
errors" and passes the unittests (including the infinite recursion
crashers removed in that rev). I have no idea about the problems it may
cause, though.