I always sharpen in a vice, and on more than one occasion the file has skidded off the cutter and ended up opening a finger up to the bone while doing the right cutters. Have done the same thing on the right cutters and punched the dawgs, full force. Gone into the webbing between my fingers and out the other side, into a vein on the top of my hand, and, worst one - punched fully in the centre of the knuckle of my middle finger, actually fracturing the kunckle :-(
I wear welding gloves when sharpening now. I usually have to do 6-12 saws a day, and it's only a matter of time before you skid off after sharpening a few miles of chain. Especially if you sharpen in a vice with a fresh file and push as hard as you can. Especially if you sharpen at the end of the day and it's been a long day, you're fatigued, not on your A game.