They NEVER use reverse thrust in flight. It's actually forbidden in most (if not all) flight manuals and perhaps by regulation and certainly stupid. If a reverser should fail to come out of reverse in flight it could be VERY bad. Reversers DO occasionally deploy inadvertently in flight and that is subject for immediate repair or the system is disabled until repair can be made to the system. My boss made a point of telling me where I could pick up my FINAL check if he ever heard of me doing it with HIS plane.[:@]
Note: I did have one stick in reverse during taxi so I know it can happen. A cable broke.