Happened just last week. In descent from 350, passing through FL120 realized the cabin was shooting past 0 feet...over-pressurizing in a big way ! Went to manual DC and toggled the outflowvalve until able to equalize differential. On the ground found that the outflow valve heated seal had failed...this is not an item that is usually checked...even during D checks...but Boeing overlooks it...in winter very very cold operations watch out for this because it can get you.
.............forgot to mention this was preceeded by AUTOFAIL light on the pressurization panel................