In bpo-36301, in commit f72346c47537657a287a862305f65eb5d7594fbf, a couple possible uses of uninitialized variables were introduced into Python/preconfig.c.

In particular, in _PyPreConfig_Read(), along an error-handling path, the `init_utf8_mode` and `init_legacy_encoding` variables will be read uninitialized.

    _PyInitError
    _PyPreConfig_Read(_PyPreConfig *config, const _PyArgv *args)
    {
        /* ... */
    
        if (args) {
            err = _PyPreCmdline_SetArgv(&cmdline, args);
            if (_Py_INIT_FAILED(err)) {
                goto done;                                            /* ERROR HANDLING DONE HERE */
            }
        }
    
        int init_utf8_mode = Py_UTF8Mode;                             /* VARIABLE INITIZLIED HERE */
    #ifdef MS_WINDOWS
        int init_legacy_encoding = Py_LegacyWindowsFSEncodingFlag;    /* VARIABLE INITIZLIED HERE */
    #endif
    
        /* ... */
    
    done:
        if (init_ctype_locale != NULL) {
            setlocale(LC_CTYPE, init_ctype_locale);
            PyMem_RawFree(init_ctype_locale);
        }
        _PyPreConfig_Clear(&save_config);
        Py_UTF8Mode = init_utf8_mode ;                                /* UNINITIALIZED READ HERE */
    #ifdef MS_WINDOWS
        Py_LegacyWindowsFSEncodingFlag = init_legacy_encoding;        /* UNINITIALIZED READ HERE */
    #endif
        _PyPreCmdline_Clear(&cmdline);
        return err;
    }

Fixed by commit 6a8c3139ae9ada89d4a95985ec7cf8bb7d03bc01.

Thanks for your bug report and fix Brad Larsen ;-)