Norm gives his empty mug to Sam. A full mug is front of Norm in the very next shot.
Cliff has an appendectomy, but right after he is released from the hospital he wrestles a man to the floor to get the button to the shock apparatus. That couldn't happen without pulling his stitches.
When Cliff returns to Cheers to be zapped by the aversion-therapy doctor, Cliff's first response to being zapped (in the bar) comes slightly before the doctor actually pushes the button. It highlights how good Ratzenberger because he must perform his own, self-administered, "shocks."