99.9% of shower pan failures happen at the top of the curb due to failure to wrap the curb and the inability of the tile man to properly fold the pan and the waterproofing membrane. Some of the crap I’ve seen and replaced could have easily been avoided like Mike said. But then again I’ve seen pans nailed to studs below the shower drain head. I tore out one where the jack leg installer didn’t know how to fasten the pan to the drain flange so he just cut out a big circle and laid the pan over the drain.