If g_simple_async_report_error_in_idle() is used, the source_tag of the result is NULL.
If you then pass in the _async function as the source_tag (as done in the example in the documentation) into g_simple_async_result_is_valid(), it returns FALSE because the following line fails:
  return source_tag == NULL ||
         source_tag == g_simple_async_result_get_source_tag (simple);

I think the NULL check should instead be done on g_simple_async_result_get_source_tag (simple) and the documentation should be updated.

This problem came up in gvfs:
https://bugzilla.gnome.org/show_bug.cgi?id=720743#c3

Thanks