My favorite has always been a panel of two snakes lying side by side in a bed together. The one on the left is asleep, and the one on the right has had a knot tied in him, and he's yelling "Charlie Horse!"
Snakes were another common Far Side theme. I remember one where an elderly, wrinkled snake is talking to two younger ones and it went something like this: "Well, it was a long time ago and I was sunning my self on old highway 37" and the old snake had to flat spots on his body that resembled tire tracks.