I thought that if you put your penis in a girl, she automatically would get pregnant.
I also thought that in deep concrete swimming pools, there was a giant whale or squid in the middle of the deep end at the bottom, and if I swam over it, it would pull me down and kill me. This might be why I like pictures with giant creatures. And after I got past that fallacy, I thought if I swam down above the white grate/drain in the bottom of the pool, it would suck out my guts.