I was told by my wicked mother that the canned laughter on TV shows was actually the sound of the audience at home coming through the TV speakers. This was plausible since we had an ancient television (replete with veneer side panels and white plastic) with a massive speaker on the side of it.
I spent years trying to hear myself laugh on TV, and as I never seemed to be able to, rapidly started developing more and more extreme ways of standing out until it reached the stage that I was practically on my knees in front of the TV making the 'sooeee' type sounds of a pig. It was unforgiveably late that I eventually realized I had been duped. My mother remains unapologetic.
So....stupidest thing you thought to be true when you were young.....