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Most people use a ConcurrentLinkedQueue or CopyOnWriteArrayList to collect listeners and notify them when something happens. The drawback of that is that it is possible that a listener can get a notification while it already unregistered himself. This happens if the manager (collection, …, give it a name) is iterating over the collection of listeners and some listener unregisters himself during that iteration.
So, the ‘remove listener’ contract is broken. Would you consider that as a problem?
Even if you checked the listener was still registered, in a multithreaded environment you still couldn’t be sure it was still registered by the time you call it. Even for non-multithreaded situations, the standard implementation is to work on a copy.