The best part is, the machines get their knickers in a twist so often that they now have pretty much exactly as many employees standing around the self checkouts to babysit customers and unjam the machines (and watch for theft, I'm sure) as they had normal checkout cashiers in the first place.

There's a grocery store near where I live that is like this. They have a scale built in to the bagging area, and it's own idea of how much your stuff should weigh, and if you don't add items precisely when it expects you to it shouts "UNSCANNED ITEM IN THE BAGGING AREA" and halts until an employee assesses the situation and releases you to scan more stuff. But this happens so often that the employees don't have time to check why it's complaining, and just scan their card to shut the thing up.