PyErr_NormalizeException may not set an error, so convert the PyErr_SetObject

call on hitting the recursion limit into just assigning it to the arguments provided.

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Fix a problem in PyErr_NormalizeException that leads to "undetected errors"
+  when hitting the recursion limit under certain circumstances.
+
 - Issue #1665206: Remove the last eager import in _warnings.c and make it lazy.
 
 - Issue #4865: On MacOSX /Library/Python/2.7/site-packages is added to

Python/errors.c

@@ -225,7 +225,15 @@ finally:
 	tstate = PyThreadState_GET();
 	if (++tstate->recursion_depth > Py_GetRecursionLimit()) {
 	    --tstate->recursion_depth;
-	    PyErr_SetObject(PyExc_RuntimeError, PyExc_RecursionErrorInst);
+	    /* throw away the old exception... */
+	    Py_DECREF(*exc);
+	    Py_DECREF(*val);
+	    /* ... and use the recursion error instead */
+	    *exc = PyExc_RuntimeError;
+	    *val = PyExc_RecursionErrorInst;
+	    Py_INCREF(*exc);
+	    Py_INCREF(*val);
+	    /* just keeping the old traceback */
 	    return;
 	}
 	PyErr_NormalizeException(exc, val, tb);