Spectating a vessel that undergoes a staging event while throttle is up (possibly while throttle is zero as well) will result in the throttle setting being stuck at that setting. The next full vessel update resets the fuel levels but not the throttle setting. If the active ship has cut throttle, the spectator's throttle setting will try to jump to zero but instantly be reset to the staging throttle value.

To clear this issue, the spectator must go back to the Space Center then re-spectate from tracking station, or the active player must warp forward a few seconds and the spectator sync.