Idk about you, but when I use self check out I scan and look at the screen before anything goes in my bag. 1 item or 50, doesn't matter. Takes 2 seconds to check the screen before proceeding. It's not rocket science. Scan, look, bag, repeat. Also, if you're paying attention, you can call over an employee and they can remove the item that was not supposed to be there.

Also, if you have so many items you probably should just go to a regular register anyways...

Source: I am customer who doesn't want to pay more than is necessary to for the stuff I need.