Wonky wheels?

Grocery shopping can be a real pain at times, with a wonky trolley it becomes a nightmare. Fear not though Sainsbury's have come up with a really simple but useful idea to solve this. If you get a wonky trolley stick a tag on it. The staff see it and take it out of service. Thinking about it now, I can't understand why I've never seen this before. A simple, effective idea that's well executed.