Sounds like operator error. It's not hard to scan your item and then put it in the bag. It's that simple. Hell, they even make the fruit and vegetables easy for you by letting you search by type and give you pictures to help you identify it.
Now, yes, the fact that some stores are run too shitty that they can't adequately staff the self checkout to take care of issues when they do arise is very annoying. Wife and I were at Target the other day...4 self checkouts. Absolutely no one to help with issues. We were 4 or 5 people deep in the "line" and there was a guy there waiting for someone to clear whatever the issue was. He was still waiting on help by the time we checked out.
Btw, was just in London for business, went to the local Tesco. All self-checkout with the exception of 2 registers...and the only time they staff those registers is on the weekend. They had 2 people (from what I could tell) that were helping with the self checkout, if need be.