The following procedure leaves an NMActiveConnection around for a deactivated device, which causes errors in libnm-glib clients when they cannot create teh GObject for the non-existent device of the AC.

1) allow a device which can assume connections to be activated
2) stop NM, which should leave the device's IP configuration up
3) start NM and allow it to assume the device's existing connection
4) remove the device, either by unplugging it or 'rmmod'

The device is removed by nm-manager.c::remove_device(), but the device object is not moved to UNMANAGED state, leaving the NMActiveConnection completely unaware the device has gone away.