The worst one I've run into appeared to be cobbled together from two different systems that barely talked to each other. At some point the main unit passed control to the other without any indication it was doing so, or that different instructions would be appearing on the smaller, monochrome screen that never showed up on the main one and had to be followed before following the next step on the main screen (no audio cues for anything on the secondary screen either, as far as I could tell, so anyone with vision issues was SOL). I don't know what the hell the designer of this monstrosity was thinking, and I don't care to know, but my cat could have done better. It wasn't impossible to figure out, but it required time and brain cells that I wouldn't have needed at a manned checkout. (Name of the store withheld to protect the guilty, but it was a major North American chain.)