(Noticed while debugging something else.)

If you instantiate a GDBusConnection with a specific address, and ask it to perform authentication as a server or allow anonymous auth:

  conn = g_dbus_connection_new_for_address_sync ("badger:mushroom=snake",
        G_DBUS_CONNECTION_FLAGS_AUTHENTICATION_SERVER,
        NULL, NULL, &error);

it hits this assertion failure:

GLib-GIO:ERROR:/tmp/buildd/glib2.0-2.30.2/./gio/gdbusconnection.c:2607:initable_init: assertion failed: (connection->initialization_error != NULL)

due to a mistake in error handling (it should have set connection->initialization_error, not *error).

Strictly speaking, this should be a g_return_if_fail or a g_critical or something (it's programmer error), but that's harder to do because we're under a lock. The other possibility would be to turn it into an assertion, or a pair of assertions:

    g_assert ((connection->flags & G_DBUS_CONNECTION_FLAGS_AUTHENTICATION_SERVER) == 0);
    g_assert ((connection->flags & G_DBUS_CONNECTION_FLAGS_AUTHENTICATION_ALLOW_ANONYMOUS) == 0);

Arguably, the following assertions should be a g_critical or a g_return_if_fail too:

    g_assert (connection->stream == NULL) if address != NULL
    g_assert_not_reached ()               if address == NULL, stream == NULL
    all assertions about flags
    all assertions about guid

but, again, that's hard to do here.