I really did used to run from the loo & get back downstairs to the frontroom / into bed before the flush finished. No idea why. Probably my big sister told me to. Probably originated in that Pete'n'Dud sketch then. I think the devil was supposed to catch you if the flush finished before you got back
Mind you they do make some scarey noises after the flush finishes - so perhaps its a fear that lots of kids develop & that the sketch was picking up on - actually that seems more likely for their humour.
Nowadays my superstitions seem to be more along the lines of the feeling that if I do something bad then something bad will happen to me in return. And serve me right too!

Kris