In flight reverse
On the trident we used in without any problems although it wasn't very pleasant down the back.
Think the limit was around 11,000 but we were allowed to use emergency reverse in the flare.
The dc8 was reverse on inners.
In the mid 70s Concorde had an engine go into reverse in cruise. It returned to lhr, got another crew and after trouble shooting was sent on its way only to have the same fault after rotation.
It crossed over my cottage, near to Newbury, so low that I rushed outside thinking there was a crash; about 40 track miles; it only got above three grand crossing the Bristol Channel. Iirc the engineeer got some sort of award for shutting the engine down velly quickly.