Simple: if you have an event handler function name assigned to the corresponding object's (gui, etc) property, but there's no real function (e.g. you deleted it), there will be no error or warning shown neigher during compilation or during game run (even when you interact with an object).
As I remember, it had to be done this way because the import of old 2.72 games would sometimes generate this sort of situation in the room scripts. However, it would be a good idea to add an option for new games to enable this checking.