Got frostbite many many years ago on ops with the Norwegian army. We were well above the snow line on skis in temps down to -25 and out for a multi day trip. Must have let my face mask slip at one point without realising it and it left a crescent shaped piece of flesh exposed below my goggles. Only spotted after we'd bivvied up for the night, thankfully was only very white and hard, recovered after a few hours in the relative warmth of the tent.