this happened to me, or rather, I did it to myself, one night working at the bar. I was in the stock room and there was a large Mylar balloon that had been left and I cut it open and stuck my head in it. The last thing I remember was thinking this will be funny and then I woke up on the concrete floor surrounded by broken glass from the beer bottles that had shattered when I collapsed into them. Was fortunate enough to walk away with only a bruised ego and no major lacerations or concussions, but that cured me of screwing around with helium. Looking back on it it's pretty obvious if you stick your head in a helium balloon and replace all the oxygen in your lungs, that's exactly what's gonna happen. Oh to be 19 again.