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Different strokes for different folks.
If the cam band's too loose, the tank can slip out. If it's properly fastened the odds are very much against it, regardless of how high or low the band is. I did lose a tank that way, once, diving with unfamiliar rental kit (still my fault, of course) but it was nothing dramatic. It was only the slight pulling of the reg that alerted me to it. Mind you, I wouldn't fancy it slipping out halfway up the boat ladder...The tanks I have seen are to the point that it looks like is it was bumped or jarred a little it would slide right out. I see it most with single cam bands. I figured if it did then it would rip the reg out of the person's mouth and hang with the lp hose on the BCD.