There was an unsettling short from the vintage era (early 70's, very early in the show's run) that seems to have vanished into the same rip in space and time as "Cracks" got lost in for so many years. I've trawled the episode guide for that period, and can't even find the sketch's "official" name in the SS archives.
It was a live-action segment about making bread. Two or three people, possibly a child helping an adult. Not a big deal, right?
But: everyone kept leaving the room for the yeast to do its thing. The film would then go into time-lapse mode to show to the dough "riiiising...and riiiiising..." as the child says in creepy voice-over, while a sound-effect plays that others have described as "slowed-down monster growls" and even "Godzilla's mating call" (!)
I'm not sure what child-me thought was happening, but some evil black magic seemed to be informing the process. They might as well have played "Ave Satani" from The Omen. How the sketch ended, or what their freaky Zool-summoning bread came out looking like, I have no idea; was probably hiding under the coffee table at that point.
Only one or two posters on these types of threads seem to even remember that this thing happened.