I'm thinking I've done that many times... if there is still a round in the chamber and you expect it then most of us would note that it ejected or not... but some guns do not lock open on the last round or some that are supposed to don't sometimes and so it would be easy to drop the mag and wrack the slide and just assume the chamber was empty when you did this. Before this discussion I thought sticking the finger in the mag well from the top when clearing the gun was kind of silly ... now I know different.I think the bigger problem is guys just working the action thinking a round would eject. They don't even look inside.
A malfunction could leave you with an unsafe situation with one in the chamber.