When it happened to my snake, the skin was flaking off and the back half looked really wrinkly. I was certain it was a retained shed but I couldn't get an edge at all. After much soaking, and upping the humidity of the enclosure by a lot, he went blue a few days later and shed whole. BUT, even after a fresh, whole shed, his skin remained dry and wrinkly and it took a few sheds more to smooth out.

This is how the skin looked. You can see exactly where it smooths out in the front:


Skin flaking off in scale-sized bits:


I'm still thinking it could have been a retained shed that caused the skin underneath to somehow stay wrinkly since there was such a huge difference from the front to back. Could a retained shed damage the fresh skin underneath?