But the point was: what if you wanted the behavior to be different than what you described. What if the intent was really "increment"?

For example, say you have a list AND a counter. Everytime you add an item to the list, you need to increment the counter, as an invariant of your system.

Of course, it's a contrived example, but you get the point: things can get more complicated and the system may break as a result, unless you're very careful.