Have you ever had a completely illogical assumption that you just can't get rid of?
When I was younger, about 13, I read every Lurlene McDaniel book that had been published: Too Young to Die, Sixteen and Dying, One Last Wish, Time to Let Go, Someone Dies, Someone Lives, et cetera. They were all about kids dying of some kind of disease, usually a cancer of some sort. In one of the books, one of the kids had the symptom 'unexplained bruising,' and it turned out to be a sign of leukemia.
Because of this, every time I look down and see a bruise I don't remember getting, my first thought is, "I have leukemia." I know that this is not a logical response. A better response would be, "I probably walked into something again," (because I walk into things all the time - I'm not very graceful for a dancer) or "I must have inherited my grandmother's incredibly sensitive blood vessels" (if you looked at her too hard, she would get a bruise). I've had this response for about fifteen years.
Today, I looked down and saw a cluster of little bruises on the side of my knee, and again, my first thought was, "I have leukemia." Sigh. Maybe one day, I'll get rid of that response.