My brother was on a rescue squad when he was in college, they got a call from a hooker one night. She was having sex with a guy who weighed over 400 pounds, he had a heart attack and died on top of her and she couldn't get out from underneath him.
Just two of the many articles available on deaths by auto-erotic asphyxiation:
http://en.wikipedia.org/wiki/Autoerotic_asphyxiation
http://www.prin.edu/users/els/departments/poli_sci/articles/LAWAA.HTM