I don't quite remember which, but it was one of the Simpsons halloween specials, and quite frankly, the reason I stopped watching Simpsons reruns altogether.

In the short where Homer enters the strange and mysterious '3rd dimension', several of the townsfolk come to try to help. Among them is professor Frink; he whips out a blackboard and says 'He's disappeared into the 3rd dimension!', then proceeds to draw diagrams playing off the idea of the hypercube, how there is a 3rd dimension which can't exist in reality, but has been proposed theoretically. It's physics. It's funny; the first time I saw it, I laughed my balls off.

Then, I saw the episode in reruns. I hear 'He's disappeared into the 3rd dimension!', then there's an obvious edit, and the blackboard just appears with no explanation, and Chief Wiggam jumps in with the line 'enough of your borax, poindexter, it's time for action!'. No hypercube. No explanation.

I thought it was a fluke. I saw it again, and a third and fourth time, but my favourite joke ever was never to be seen. And then, I started noticing other episodes which did the same thing, just to save a few extra seconds for commercials. I wept, I frothed, I was angry, I stayed up nights and wrote letters to Fox, but no avail; my joy was never to be found again.