I'd never think to harass the poor employee who has nothing to do with the store managenent's decisions...
However, when I'm pissed or tired I'll sometimes be rough or sloppy with the machine, and I get pissed if they have too few manned checkouts for how crowded a store is. Banging items against the scanner glass, tap selections on the touch screen forcefully with my ring etc.
To keep the self-checkout machines company, I'll act like a machine too. If I unsuccessfully attempt to scan something, after 5 tries I "timeout" and move onto the next item.