People getting in will cluster around the button panel, forcing everyone coming afterwards into conflict
People will insist on pushing the button as soon as they get in, delaying full loading and departure
People will usually not queue up while waiting for an elevator to arrive - the sides of the main line will always get full of people trying to slide in.
People will try to get on the elevator before people have finished getting off.
When you are going up, frequently people waiting for a down elevator will get on by mistake. Then they'll see it is going up and mutter. This is despite the fact that the elevator call button they used is still lit.