I don’t know of biology students habitually getting anything wrong about snake anatomy, but I bet most would be surprised to learn that snakes have two penises. Each is called a hemipenis, and conveys sperm from one testis. When mating, males can use one penis after the other, but only one at a time. This enables a male to inseminate one female after another, if he gets a chance, while a testis depleted of sperm from the first mating can replenish itself.

Hemipenes of a black rat snake