My experience is from Norwegian shops, so it might differ from the standards in elsewhere.
- item won't scan: very rarely an issue. We can put in the bar code numbers manually if needed.
- you have alcohol: a nearby staff will take a quick glimpse at me and will approve the age from their unit. Only people close to the age limit will have to wait for the staff.
- discount stickers in shops here have the updated price on a barcode on that sticker.
- I don't understand the "2 upcs"-issue.
- accidental scans can be corrected by the user without staff involvement.