We have a few APIs in GIO now, particularly on GFile that are unbindable.  For example:

void g_file_copy_async (GFile                 *source,
                        GFile                 *destination,
                        GFileCopyFlags         flags,
                        int                    io_priority,
                        GCancellable          *cancellable,
                        GFileProgressCallback  progress_callback,
                        gpointer               progress_callback_data,
                        GAsyncReadyCallback    callback,
                        gpointer               user_data);

and the new:

g_file_measure_disk_usage_async (GFile                        *file,
                                 GFileMeasureFlags             flags,
                                 gint                          io_priority,
                                 GCancellable                 *cancellable,
                                 GFileMeasureProgressCallback  progress_callback,
                                 gpointer                      progress_data,
                                 GAsyncReadyCallback           callback,
                                 gpointer                      user_data);

These are unbindable because the scope of the progress_data argument is weird.  To a C programmer, it's very clear that you will get progress callbacks until the async operation is complete, at which point they will stop.  We should teach gobject-introspection about this by way of some sort of (scope asynccall) annotation.

ie: the progress_data will be freed at the same time as the user_data that was marked (scope async) -- when the async call is delivered.