Well, sometimes the monsters follow you the rest of your life...or you acquire new ones as an adult. What irrational (or quite rational) worries have you acquired as a result of reading? Have any of them ever kept you from making mistakes?
Tiny Little Medical Problems: I've been paranoid of splinters ever since reading On Tide Mill Lane. Wouldn't you be?
Leaving things too close to light bulbs: The Saturdays.
Creepy old houses, not to mention reading under the covers: The Letter, the Witch, and the Ring. You wouldn't believe how badly that book scared me one night...it cured me of after-hours reading for some time.
Running with scissors: The title, I can't remember; but it was a Parents' Magazine Press book from about 1970, about safety rules. A bit like Struwwelpeter although not so extreme. One little raccoon "liked to cut paper dolls, she snipped away happily singing" until she heard a friend's bicycle bell ringing--and ran with scissors, to what end exactly we're not sure. [Oh, look at that: I just found the title. Watch Out! How to Be Safe and Not Sorry, by Harold Longman.]