I think I've found the correct example...
Flying a descent manually with your FD off.
You take manual control whilst in "Lvl. Chg" - disconnect the A/P and deselect the current speed mode, which is "Lvl. Chg" to give you manual thrust. But deselecting "Lvl. Chg" dumps the other guy's FD pitch bar, so if you reselect "Lvl. Chg." he get's the pitch bar back and you (for the moment) still have manual thrust (A/T annunciated as "Arm"). But when the aircraft goes from "Lvl. Chg." to "Alt. Acquire" / "Alt. Hold", the A/T will advance the thrust levers - which is not what you want. So to fly with manual thrust in the descent, you have to have the A/T off to keep the other guy's pitch bars and to give you manual thrust.
Have I got the right end of the stick here?