This wasn't a lie told to me by my parents, but when I was about six, my two older brothers (five and six years older than me) decided for some reason that I was old enough to learn about sex.

I can't really recall the part when they were explaining the mechanics of it all, but I distinctly remember them saying at the end that a) if I did this before I was eighteen, my brain would fizzle up and then I would die, and b) this is why I should listen to our mom when she tells me not to sit with my legs spread open, because there's the chance that an ant could crawl up there, and then the whole brain fizzling and me dying thing would happen.

I believed that sex before the age of eighteen kills girls well into my high school years -- which, for a while, confused me because I never understood why everyone wanted to do it so badly. Also, even now, there are times when I get paranoid about sitting cross-legged or with my legs in a V, because some deeply ingrained urge is telling me to protect my hooha otherwise an insect is going to crawl up into my jazz and then I will die.

TL;DR: Older brothers convince me that if anything -- a penis, an ant, etc. -- enters my hooha before the age of eighteen, I will die. Still irrationally paranoid today.