In this same apartment I am in now I once dropped my bathroom cup in the sink (china cup, not heavy) and it didn't break...but suddenly I was shocked to see a HUGE HOLE in the sink. I looked this up online. It could have been there was a hairline fracture in the sink when they installed it...and moisture was probably seeping in the hairline crack for awhile.

I filed a work order, then went on vacation. When I came back they had installed an entirely new sink. Of course no charge to me.