No we shouldn't. I would even go further and would say if it's accessed from a different thread it should better fail immediatly with a hard error that everybod

No we shouldn't. I would even go further and would say if it's accessed from a different thread it should better fail immediatly with a hard error that everybody can see. Multithreading isn't something that should be implicit. Either you design your whole module/library/application around to be used from multiple threads (with all the extra work for synchronization and typically also well defined thread boundaries) or you stay with single threading.