In the company I fly for as a Captain on the 737NG 7/800 we had an aircraft which would not reduce thrust below +/- 60% when starting descent. It would show retard and the thrustlever would move to the idle position but the N1 would not reduce below 60% on one engine. It took a couple of manual thrust applications and retards to get the thrust to decrease to idle. It was a real headache for maintenance because it was initially verry intermittent and not obvious until you had a good look at it.