There is a scene in about the first quarter of the episode in which Tenchi scoffs at the sword he finds deep in the cave he's not supposed to enter. "Give me a break! 'The sword that cuts through a rock'?" He sneers, swinging the rusty old thing at a stone with a seal on it. Although the stone is split neatly in two, the rusty blade shatters.
One of my friends then said, quietly, "Tape." And we all busted out laughing.
That's what makes this so funny: