The best is when you buy fruit all vegetables and it decides that the Apple that you are buying doesn't weigh the right amount.

One time I bought the largest potato you've ever seen in your life and it decided that it was too big and therefore could not possibly be a potato and must instead be some high value item I was stealing, although nonetheless scanning. But the problem is the alternative is to talk to people.

I accidentally managed to rack up 36 large bottles of water. That amount wouldn't even fit in my car.