Are slides safe to use in the closed position?

Our manual says not to use it when it is in or walk on it. It sounds like it could get bent or something and not go out or retract properly. When we are traveling we usually use the rest stop restrooms and stop at a fast food place or a restaurant and walk around for awhile. Could be different in a Class A or maybe in a 5th wheel, but our trailer manual said to stay off of it. So we have and have had no problem with it. It is a Lippert.