I have looked up at what is attached, and I don't think the chairs woudl slide even if they didn't fully attact to the cable again, atleast not slide fast. Also in that case, I would be worried about the chair downhill (that gets run into) more than being on the chair that slides.
I have never heard of a case where a chair would come off the cable 1/2 way up HOWEVER, up at killington last year I was getting on a gondola leaving the station when another one came in and never slowed down, it slammed into the one infrount of that, and there was a nice little chain reaction...good for about 20 minute inspection. And its not even like there was ice on the cable or anything, it was nice weather.