You can build a socioeconomic graph of the country by how anal the unexpected item in the bagging area sensors are.
Some places will detect a fly farting on the damn scale, others can take three or four kids climbing on it before it complains.
The walmart near me apparently doesn't even use the scale at all, I had a full cart once and asked the attendant what to do, and they said just put the bag back in the cart.
The grocery store down the street though is exactly like this, gotta stack everything up on the scale to make it happy.
There is a grocery store about 2 miles from my house that will freak out if you look at it funny. I gave up one day, the helper person came back for the 3rd or 4th time to unstuck the "self"-checkout in my ~20 item shop. I told them they can just cancel the transaction and walked out. I now go to the grocery store 8 miles away, that always has at least 1 human cashier open in addition to their self-checkout lanes. I rarely use the self-checkout because they are the ones that are only useful for a handful of items, but I've never had it give me a problem.