My personal favorite is the "You swallow 8 spiders a year" misconception. That's ridiculous, because spiders love warm and moist places, making 8 a year far too low of an estimate.
I honestly don't even think that spiders crawl into your mouth while you sleep.
Wouldn't they bite you if you did anything? But if they did crawl into your mouth, then I would think that 8 a year would be a good estimate, because chances are that there aren't that many spiders near where you sleep, and most of them would have a web somewhere anyway.