So there I was, walking along with my 6D & 16-35mm II and boom, my knee gave way on some steps, I fell to my knees and the front of the lens hit the ground, sound of glass smashing as I commando rolled over... I've grazed both my knees, my left arm and dented my pride... Plus did the attached damage to my 16-35mm II - can't get the filter off at the moment, going to take it into the local independent shop when I get home from holiday, filter is a dead loss, lens looks ok bar the marks on the outer edge and until I get the filter ring detached, I've no idea of the damage to the front filter thread. Lens appears to work, two scuffs on my 6D are annoying too :-/
So folks - always fit a filter !!!