PyRun_InteractiveLoopFlags() only stops if PyRun_InteractiveOneFlags() returns E_EOF. But PyRun_InteractiveOneFlags() if an error occurs:
 - sys.stdin has no encoding attribute
 - PyArena_New() returns NULL
 - PyParser_ASTFromFile() returns NULL
 - PyImport_AddModule("__main__") returns NULL
 - run_mod() returns NULL

You can reproduce the error by adding the followg line to the end of Lib/site.py:

  sys.stdin = object()

Attached patch is for py3k, but the bug does also exist in trunk.