If you cut your own hair, it won't grow back. That was a lie my mum told me (after I experimented with the scissors). I believed her for years because there was a gap in my hairline. Eventually I realised "how would the hair know who cut it?" The gap in my hairline was just my parting.

I believed LCD screens in digital watches were made of mercury (they were silver after all), which I knew was toxic. I thought that if you touched the display directly, you'd die. One day, I'd disassembled a cheap watch to see how it worked - I took everything apart back then, eventually I got good at putting them back together again. Drove my parents mad, but these days they always have something for me to fix whenever I go round.
Anyway, I had this watch in pieces, handling the innards like an IED, but disaster! I brushed the back of the screen with a fingertip.

I was dead. It was just a matter of time. I didn't cry or run for help, nothing could be done, I was resigned to my fate.
After about an hour of continued existence I began to doubt my assumptions. It dawned on me that something so frighteningly lethal wouldn't be simply handed to children with nothing but a cheap, press fit case! That said this was in the 80s, and back the I also believed it was both safe and fun to help demolish an asbestos cement outbuilding by jumping on the sheets to smash them into little pieces. That one might still get me, we'll see.