Those machine infuriate me. "Place item in bagging area" *OK...done* "Invalid item in bagging area, please remove this item" arghhh.
I feel sorry for the staff. I hear it for maybe as long as 5 minutes a fortnight. Those poor sods hear it over and over again for maybe 40 hours a week.
My local Tesco self-service checkouts have a fondness for declaring that there is a problem with the last item, which requires a member of staff to come over and wave a card at the machine. Any relatively light item is quite likely to trigger that. I bought some sachets of hot chocolate last time (convenient for work). I won't do that again.