Metro (a daily newspaper available in London) have dispelled their top 5 urban myths. At number 1:

1. You accidentally swallow about eight spiders a year in your sleep

Why it’s not true:
Spiders have a natural instinct to stay alive and crawling into the heavy-breathing hot, wet mouths of animals goes strongly against this.

Who started it:
Back in 1993 a columnist for PC Professional named Lisa Holst decided to prove that you could make up anything on the internet and people would believe it.

She did this by making up a set of facts that were utterly ridiculous, the spider myth among them (which itself was taken from a collection of insect folklore that dates back to the 1950s, and emailing her findings around the world.

I find this very comforting, as I heard that one a while ago and it kinda freaked me out.