To put to rest any fears people might have gained from this post I have to say that I've being working with many fiberglass projects, including large fiberglass boats all of my adult life. The event described has never happened to me and I've drilled thousands of holes with standard bits and hole saws. As has been mentioned sometimes a bit will grab as it breaks through the bottom of the hole but that's true for many materials.
So I'd say, if you have a hole to drill, do it without worry, it's easier than drilling some other materials.
One trick I do for using handheld hole saws is: use a 3/16" bit or whatever size the bit is in the hole saw and drill the hole. I then use the hole saw with a 3/16" smooth rod. There is less danger, when drilling freehand, that the drill bit will work sideways and make the guide hole sloppy and provide less guidence for the hole saw. Hope that makes sense