Title says it. The verification checks whether a naval embarked unit can move into the city, ignoring the fact that the embarked unit will become a land unit when it enters the city. It should be legal for the (disembarked) land unit and the naval unit to be inside the city at the same time. Quite often this won't matter as you would move your work boat out. I've found a situation where it would matter, since I accidentally embarked one turn, the English declared war the next turn, and it's a problem. Reload!