I have had one puck go bad on me in two years for wet shaving. It was a puck of Bain Amour Southern Gentleman. Their soap is a soft artisan soap made with local beer (which I suspect may have had a hand in it). I am not sure what was wrong with the puck, but it had shrunk in the tin and had a bunch of odd colored fuzz growing on top of it. I should have taken a picture of it before I binned the puck. I am not sure if it was mold, but I didn't feel like taking the chance. This puck was stored in a cool, dry place. So, things can happen. But, this shouldn't be a frequent occurrence.
I doubt I will ever see something like that again.