On all the jet airliners I've flown, you had to be in idle reverse as the speed decreased to around 60 knots, except in an emergency such as a rejected takeoff. If you retain reverse thrust above idle to a slow speed, the engines are liable to re-ingest the hot air, leading to surging, loud banging, and possibly brief bursts of flame.
It isn't catastrophic, but clearly doesn't do the engines any good.
