Allowed isn't really the right word is it, it clearly isn't allowed, but it seems obvious that somewhere there's been a miscommunication and either the operating staff have mistaken the unit to be taken, or been told the wrong one.
Rather unfortunate but not unknown either - wouldn't be the first time a train has been "lost" - more than once I've seen group emails go out to all traincrew and depots trying to find a unit that isn't where it was meant to be due to an error some time previously.