In the episode when they crash-landed on a planet with giant women. When Fry, Zepp, and Kif were captured, a giant asked Leela what men do on their planet, when Leela whispers something, the giant says that they called it snu-snu. When the Femputer sentenes them to death by Snu-Snu, the men breathe a sigh of relief. The mistake is:

How were they supposed to know what snu-snu is? They never been on that planet before, and the giant says, "Oh, we call that snu-snu on out planet." They don't know what it is, it might be to flush them down a electrical toilet, or eat them!