A million years ago in instrument training I had the pitot ice over in IMC when I did not turn on the heat. The airspeed needle just walked around the dial to max airspeed in about five seconds. Gets your attention. There was no increase in wind noise or any attitude excursions so I just assumed it failed. When we turned the heat on the reading came back to normal. Hate to think what could have happened if the aircraft had had an engaged autopilot that was wired to pitch up when it received a high airspeed reading. Maybe not the same as what may have happened here but similar.