Today I was shopping at a grocery store I don't usually go to. I parked at the far edge of the lot because I had to go to another store first. When I came out I pushed my cart out to my car, and about 10 yards before I got to my car, I heard a clunk and one of the wheels of my cart wouldn't roll. The cart was full of groceries and I had to pick up the front of the cart and drag it. Pretty annoying. I thought I got something stuck in it until I stopped and bent down to see what was wrong.
I discovered the cart actually had a locking boot!! It had activated because it decided I had taken the cart too far from home. I looked around to see if there was a radio transmitter in the store or some sort of invisible fence. I saw one of those wire strips embedded in the surface of the road, like a traffic light sensor, running across the parking lot. Would have been nice if they put it at the edge of the lot instead of halfway across the plaza.
The cart wheel looked about like this picture.