dmnf

Yes most of the self-checkout is slow because of the general public does not know how to scan barcode.  
The scenario Tapioca mentioned is the machine suspects you cheat but it just says it nicely.  The shopping basket side and the bagging side is like a 天秤.  What ever weight on the left must be transferred to the right after each scan.  The scanner knows the weight of the scanned item.  If you scan and put the item in your handbag, the 天秤 will lose balance and it will suspect you cheat and then alarm the attendant.  Avoid doing this.  Both sides (metal platform) are a big digital scale.  

I think you can do rearrangement in the bagging area as long as you don't scan at the same time.  The machine assumes 1 person operations.

dmnf

天秤 is almost the only way to make sure people are honest.  

For Ikea, they may not use 天秤 method because of their heavy items.  For sure save-on-foods and other grocery stories use 天秤 method because the staff over there told me this story.