Fire, probably. But also, how complicated would you make the system if you needed to prevent certain switches from working during certain times of flight? At some point... we're all just in the hands of the people in the cockpit.
A friend did exactly that in a manual transmission, doing 100km/h.
She was mad and said she has to jam it hard ( going for 5th and missed), but it went into reverse. And the gearbox literally hit the road when she let out the clutch.