I did the same with my first motor. No plane, just holding the motor with my hand, a big motor. When I switched the throttle Chanel from 0 to 100%. Lucky the motor jumped out of my hand breaking the prop and launching a blade to the wall at really really high speed.
No injury but almost a heart attack.
For anything I do other then cleaning my quads If I am in the house and doing anything the props are off. Powered or not. Keeping that habit makes it less likely I will forget and power up with props as old as I am getting with senility setting in and all. It only takes a few seconds to put em back on when I am ready to fly.