There has been at least one fatality from
asphyxiation in a zippered up bivy.
Would you be able to provide any details on that incident? Even an approximate date and location?
I've heard a lot of silly things involving camping gear, like a story about someone who tossed and turned and eventually garotted themselves on the draw cord for his mummy bag- but none of those who have told that story around me has ever been able to document it. There are a lot of urban, suburban and rural legends out there.
I'm not saying it's impossible, I'm saying it is improbably. If it happened, there must have been something more going on. I don't doubt benjammin's experience, but there was a combination of factors at play.