Air embolism
A girl at school told me once that if you blew into a woman's vagina while going down on her, it'd kill her. I don't know if it's true, but I can easily imagine an episode of Quincy where Jack Klugman says "How many more teenage girls have to die of air embolisms before you start teaching oral sex in schools? Why I oughta!", so it probably has some factual basis.
written by da* up*igh*, approved by Log

I heard this chilling caution too, and recall specifically that the reason for the terrible risk of death upon blowing into the vagina was that the air gets into the bloodstream.
This explanation is delivered with a sense of gravity, and a look that said 'Ah, I knew you were going to ask me that. Most people who've never seen a lady's downbelows ask that question.'
Questioning the truth of the air embolism was therfore akin to saying "I am such a big virgin that it's gone beyond a joke. I shouldn't be allowed."
written by Ro* Yo*ng, approved by Susan

Actually, I used to repair Hysteroflators for a living (used for endoscopic surgery). If the uterus is inflated at a rate faster than 0.2 litres per minute, you run the risk of bursting it. Killing a woman in this manner, whilst possible, would involve creating an airtight seal and blowing very hard.
written by Po*glesn*tch*, approved by Log

More evidence that women are in constant danger of their fannies blowing up, from Hannah Peterson.
Going by the pages I found by searching for "vagina air embolus" it's possible, but more likely soon after childbirth. A quote from http://www.midwivesonline.com/sex.htm: "It is not advisable to have penetrative sex in the days after the birth. There is a risk of air embolus - meaning that the penis can act as a piston inside the vagina and drive air into the maternal blood stream through the raw placental site. This air can travel to the major organs and can be fatal."
I'd imagine that blowing into the vagina could maybe have a similar effect, depending on how hard and piston-like you blew through the raw, placental site.
written by Ha*nah*Pe*erso*, approved by Log

I wish I'd thought of using this myth to entice boys to get their heads down there. Perhaps we could have progressed to something a little more satisfying than the girly day-after reports of how you got "fingered".
written by li*se*cogo, approved by Phil