As a kid, I got this idea in my head: "Gum is sticky. If I try to swallow it, it will stick in my throat and I will choke and die."
I don't know where the idea came from, and as far as I remember I came up with it on my own. Strangely, even when the complete irrational nature of the idea was brought to light when I saw one of my peers swallow his gum and not choke, I still couldn't kick the idea.
Even more strangely, this idea has followed me into my adulthood, so now I'm a 26-year-old man who has a notable psychological block against swallowing gum. I've swallowed it on accident a few times but as far as I can remember I've only managed intentionally swallow it once...and that took a significant amount of willpower.
So, long story short, I spit out my gum when I'm done with it.