Like so many others, I found the Terrible Tunnel episode really creepy when I was little.
When I was little I remember being at the babysitters and thumbing through a magazine they had. In the magazine was an ad for an upcoming series on HBO. Fraggle Rock. The ad had a picture of Red's head popping out of a Fraggle hole. And for some reason, I thought Red was a monster. She kind of gave me the creeps. Every so often after that I'd get the magazine, turn to that page and enjoy the feeling of getting creeped.
Of course the show premiered and I watched it. I immediately stopped thinking of Fraggles as monsters and fell in love with the characters. To this day I don't know what five year old me was thinking. Red a monster. How utterly rediculous.