I have never lost a mask, but I THINK it may freak me out a bit the instant it is knocked off, simply because here in socal the ocean water is quite cold, and each time I do a half fill, or completely remove my mask for practice, the sudden rush of cold water hitting my face always catch me off guard, no matter how much I realize the water will be cold hitting my face. For a quick second by breathing gets off rythm, from the cold shock, and that is with me KNOWING im about to let water into it.
Now if it was to catch me off guard, by being kicked off or taken off by a seal or something, im sure it would be a bit freaky at first then hopefully I will take care of the problem. Just thinking about it happening, I think is a good thing and it sort of prepare me for the sudden surprise.