Only time ive ever actually left an instance (not counting vote abandons) was actually recently. It was around when i first hit 60, and I was doing Fractal Contiuum for...the second time? Basically one of the dps kept stepping on mines (back when they took out most of your hp, unless that was just a result of lower ilvl gear, but w/e), and the tank wouldnt get out of blue spaces no matter how many times i told them to avoid them and they wouldnt go through with the abandon