I have never seen that in all my 16 years of training clients.
With my personal training clients, I cannot even imagine how it could happen short of somebody already coming in sick and not wanting to let on. Even there I like to think that I would notice something wrong before it would manifest itself in such a drastic way.
As far as group exercise is concerned, I am teaching both water aerobics and MELT. I have seen people in water aerobics sometimes pushing themselves harder than they needed to which may have let them to get dizzy and light-headed but never more than that.
If I had to assume that I was the cause of such misery, I would have to question my ability as a personal trainer.