I have noticed this most recently in the A6M2, but I suspect it happens in all aircraft.
If you mess around with RPM (using the keypad "+" and "-" key) and throttle enough, you can get to where you can no longer seem to reduce the engine RPM anymore. I can reproduce it doing the following.
1. Take an A6M2 Zero up to 11k altitude (use wind layer to get up there fast).
2. Engage autopilot for straight and level.
3. Start reducing and increasing the RPM and throttle. Tap the "-" key a few times, wait a second, hold down the "+" key. Wait a couple seconds. Hold down the "-" key for a while. Tap the "+" key a few times. Move the throttle setting. Hold down the "+" key. And so on. Basically, randomly play around with the "+" and "-" keys at various throttle settings.
At some point you will find that you can no longer reduce the RPM very much at all. Or, it will reduce quickly from 2500 to 2300, then (without you doing anything to the "+" or "-" keys) drift lower and lower over the next 10's of seconds.
I found this while trying to optimize fuel usage during the Rangoon '42 scenario. It was very distracting. Also, I think the fuel usage is being computed as going down when the RPM gauge shows fairly high RPM (as if the keypress counted for fuel computation but not for gauge appearance).
