What is it about U Scans that is so hard to figure out? I get it that the first time you use one it may be a learning experience but after that what is the problem?

Last week we were at the market and a guy was having problems with the U Scan, so many problems that he tried to leave without paying. The woman went to tell him he had not paid and he just left his cart in the middle of the electronic door when he went back in.
The door closed on the cart several times as people were coming and going, each time having to walk around his cart to get in the door.
What is the problem? Why not take your cart back inside while you finish checking out?
I would surely know if my sale did not go thru and was walking out with free groceries.
M