"such a simple thing but a really unpleasant thing to happen if your mask gets knocked off and you lose it."
Happened to me once on a night dive, was sat with my back to the swell sitting on the coral walk-out, putting on my fins (had my mask on but pulled up onto my forehead).
Wave comes over me and knocks it off. Turned out to be a bit awkward as we were in restricted water and a night patrol came along. They had a big searchlight on the back of the jeep and there I was in a bright orange horseshoe :)
I had to deflate and let my buddy guide me down to 10m whilst we watched the searchlight playing on the surface of the water, whilst the other two carried on looking for the mask.
Damn near shat myself, but it was also exciting, especially when you are only 17 years old on one of your qualifying dives ;)