Take the traditional door knob. You have learned through experience that the door knob is a device which is grasped, rotated, then pulled to swing a door inwards. What if the door knob was put on a sliding door? You would first grab the knob and twist, and it would not rotate. You would then pull the knob, and the door would not open. You would probably stop at that point and reconsider the ways the door could function, but you are immediately presented with a situation which is contrary to every experience you have had throughout your life up until this point.
How about car doors? They are flaps or handles on the door of a car which allow for grasping with the fingers. You place your hand under or through the handle and pull, and the door opens. Would you put that handle on a push door?
Now picture a fire door. The handle for this is a bar across the entire width of the door, attached at approximately a 45 degree downward angle. It is obvious to all that this is supposed to be pushed, and is designed that way. Would you put such a handle on a door which was to be pulled towards the user? How about a sliding door? Would you put a car door handle on a fire door?
You take these things for granted because you experience them every day. They have, however, been carefully engineered for purpose, and using them in situations for which they weren't designed can absolutely lead to injury or even death. Just from door handles.
Now, then. Tell me again about software UI design and how people shouldn't be confused when there are changes.