I have. Not in a saddle but with a harness. How far you fall depends on how much slack is in your line. I was setting a hang on, had about two feet of line from where the line was touching the tree to where it was clipped to my harness, my foot slipped off the rung of the ladder I was standing on. I fell about a foot and then slammed into the tree and skidded to a stop, my face against the tree broke my fall perfectly. Smashed my glasses up, had a black eye and picked bark outta my face for awhile. FAR MORE PREFERABLE than falling out the tree.
So if you have alotta line played out you might fall a bit further, I suppose in a rare circumstance you could fall/slide all the way down. There's a video on here somewhere that shows a lineman falling during a competition and he literally falls/slides from where he was standing all the way to the bottom. I try and keep mine so I'm as close to the tree as possible, letting slack out to climb and then cinching back down. One thing I've seen some guys do is wrap the line once around the tree. Then when they slack they just move the loop up the tree. In theory that'd certainly arrest your fall but I think it's overkill and a pain to climb that way.