Well, there have been some cases of death from asphyxiation sex - a considerably larger number than the well-known ones that have ended up in the news - and it's safe to say a few of those would have occurred during careless s/m playing (a plastic shopping bag tied over your head while you're bound can be lethal, especially if you're gagged and/or stoned too):

There was a murder case in Stockholm, Sweden a few years ago when a man had, it seems, agreed to be dommed by a girl of the light guard at his home. He had been handcuffed to a chair, taunted and "interogated". Then the game went out of hand. They had both been drinking hard before playing which is not great, and apparently (from neighbours testimonies and police reconstruction) the girl brought a carving knife into the play, stabbed him deep in the chest with it till he died and then robbed him of some money and started a fire to hide the facts (she claimed someone else had come in and killed him, while she tried to protect him, and then this guy had escaped - but agreed it was she who had handcuffed the man; her fingerprints were on the knife). A Stockholm dominatrix actually was called as expert witness at the trial.
The girl is now serving a life sentence for murder.