When there was no line, to save walking uphill.
I have but I feel like an original gangsa when I do it. (Original Gansta=OG=Old Goat, get it? Ha, ha.)
(OK it's not a real poll).
I saw a lot of people poaching the SS line over Christmas. I generally let it go, but have recently started politely asking the obvious poachers "who are you skiing with?" It's rude to help yourself to an obviously shorter line when the rest of the maze is full. I don't buy that they didn't see the sign or thought it was the singles line, especially when you see the same people doing it on multiple runs.
Yes, once, when some foreign family of seven (couldn't recognize the language) was parked all spread out across the next four entrances. There was nobody, not a soul, in the ski school line but the liftie/traffic cop was all over us before we could even duck under the rope to move into the nearly empty regular line.