The scene in "The Bells of Fraggle Rock" where Gobo finds all his friends frozen solid. (What's more, it's the second time his irrational actions have caused problems, the first being the blackout in "The Day The Music Died"...I love Gobo as a character, but if he had messed up a third time, I would've left him to the Gorgs!)
It was a pretty upsetting moment for me; at age 3, you do worry if that could have been the end of the characters (of course it's not as bad now once you remember Doc has a blowdryer, and that if Gobo could get control of it and enough extension cords, he could have had the core of the Rock thawed out in a couple of months, possibly (you do have to wonder if the cold extended outside in the Gorgs world, and if Gobo would have found 3 giant Gorgcicles standing around if he'd gone there as well)).