[Taken from ‘Pilot Workshops.com’ https://pilotworkshop.com/tips/surprise-propeller-motion/] 

One day a while ago, I jumped into my airplane for a quick local flight and was shocked when I turned on the master and the prop started turning. I shut off the master right away. Fortunately, nothing – and more importantly no one – was inside the prop arc. This happened because the starter contactor (which can also be called the starter relay) had failed in the ‘on’ position. Despite the fact that the airplane keys were still in my pocket, turning on the master powered the starter and the prop immediately started turning.

Because of this, I’ve added a ‘Prop area – CLEAR’ item before ‘Master – ON’ to my prestart check to ensure that if this happens again the possibilities of damage or injury are minimized.

Many of us have the essential ‘prop clear’ on a checklist right before turning the key, just like we have a check for oil pressure or ammeter not full scale immediately after engine start. However, few pilots think twice about turning the master on when they first open the plane to check fuel levels, deploy the flaps, or do any of several other pre-flight tasks. Don’t be one of those pilots. Any time you energize a system, ensure people and objects are out of harm’s way.