Since I was doing multi-day dives at one shop, they told me I could keep all my gear together in the same dive bag. I put my personal prescription mask in there, without putting the mask in a plastic hard case. With the way the dive bags are typically transported (tossed, stacked), the mask eventually got damaged - the clips that held the lenses in place cracked. I did notice the potential problem and asked the guides to carry an extra mask for me. Sure enough, upon entry one day, one of the lenses came apart from the frame. I was able to hold the lens in place and signaled to the guide, "trouble... mask". He handed me the spare, I swapped out the mask underwater and continued the dive, though my distance vision was definitely lacking. Back at the dive shop, they happened to have some masks that people had forgotten. One of them was good enough to give me decent vision for the rest of the trip.
Lesson: Always keep the mask in a hard case.
That particular mask always leaked a little bit (maybe due to mustache), so I had plenty of practice clearing. By extension, it was pretty easy to don the spare mask and quickly clear it without being panicked or fussed.