Pitot Ice
Pitot probes have a liquid drain (bleed) hole, whose air bypass has to be accounted for in the airspeed calcuation. Clog the drain, and pitot pressure rises, giving erroneous high airspeed. Indeed, per reading in this thread, some of the earlier Airbus pitot malfunctions were with poorly manufactured drain (bleed) holes.
Add more ice, and you clog the pitot head itself, and then pitot pressure will lock, or will decrease if there is any drain opening at all.
GB