I stopped in a supermarket on my way home Friday night and when I was checking out the clerk erroneously scanned my single item in with the stuff the customer ahead of me was buying. I would have expected this to occur fairly often and be easy to fix but apparently a manager was required and none was immediately available. Meanwhile another line opened up so I was able to switch and complete my purchase but the unfortunate customer ahead of me was still waiting as I left.
This seems like a surprisingly poor design particularly for something as heavily used as supermarket scanners. Of course you need to be careful not to provide options that make it easy for the clerk to steal but I don't see the risk in allowing the clerk to cancel the last item scanned. I suppose it is possible the clerk just didn't know the procedure for fixing the mistake but I got the impression that the problem was they lacked the authority which seems like bad design.