Yes, I once saw the tachometer "hovering just over Mach1". Round about 40K East of Greenland heading West. Paying insufficient attention we found ourselves in mountain wave. Poor old autopilot trying to maintain height in a very strong up flow. Throttles closed, airbrakes out. Then we got into the downside of the wave. It took a little while to get back to straight and level! And this was in a perfectly standard tanker.