I once got a Miru in a shipment from Ebay that had the bottom half completely broken off. Everything below the 'nose' was gone, so I can't use it for standard MoCing purposes.
I also got two Noble Mahikis; one tan, one orange, from two different people on Ebay. On both of them, the right side of the mask is bent inwards.
Tahu Mistika's mask got slightly bent but is still useable; I speak of this mask because the stresses portion is very visible.
One of my gold Avohkiis has a very weird expression now; it looks like it is raising an eyebrow.
The grey Hau that comes with the Ussanui was bent when I took it out of the box. There were no stress marks, which means that it was molded that way. Fortunately, it still fits on a Toa head, albeit slightly crooked.
One of my original red Hau's is slightly bent around the eyeholes.
My gold Vahi has stress marks, but is not bent and is perfectly useable.
Then I have the general amount of scratches and scrapes on my older masks from when I was younger and thought nothing of the value of the sets. I would take a figure, set him on a tree branch, pull it back, then let go and watch him fly. Unfortunately, I always used Brutaka's sword blades as wings, and I broke several that way. I know these aren't masks, but they're still broken.
My dad once backed over a white Mata body on accident. Surprisingly, I can still use it, and make Kopaka appear as if he is twisting to the side.