Some aircraft are certified for in-flight reverse, as some above have stated. Usually this is in place of Spoilers/Speed Brakes.
For the aircraft that are not certified for in-flight reverse, it would take a series of failures for it to happen.
At low speeds however, it is not as devastating as you might think, and it is practised in the Simulator all the time. That is, speeds below 250 knots.
As it was considered very unlikely, jet aircraft were never tested at high speed reverser deployments. However, the Lauda Air B767-300 that had that problem above 300 knots resulted in, as we all know, very bad results. As a consequence, aircraft are now tested for reverser deployments at all speed/mach regimes.
The heading picture of the B747SP, appears to have an unpainted part, not a reverser deployment.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!