The tp_clear handler of the module object calls a custom clear function if the PyModuleDef.m_clear field is set. This function can set an exception which will be leaked to the garbage collector. An exception in tp_clear is not expected and caused a crash in the garbage collector. In the master branch it will cause just writing a traceback to stderr (see issue33622), but in any case it would be better to handle the failure locally in the module's tp_clear.