Reverse thrust lockout

[littco]

I can only comment on the aircraft I fly.

The thrust reversers can only be deployed when the primary throttle levers are in the IDLE position and the aircraft is on the ground. Landing gear squat switches prevent the system being energised when airbourne, there is one on each landing gear and BOTH have to activated to work the system.
In the event of an inadvertent thrust reverser deployment, the throttle feedback machanism will move the affected engine's throttle to idle automatically.

There are also emergency stow switches on the glareshield.. And finally the hydraulic system is a fail safe system, in that should anything fail on the associated hydraulic system, actuators or switches the thrust reversers will fail to the closed position.

This is one type on a specific aircraft, and as only given as an example of how an aircraft may be configured and is by no means meant to be a reflection of any or all there types. I think it will just give you an idea of what safety measures can be employed to prevent such an emergency situation.