I've seen this a few times with my own eyes, and it's always been when a C-17A has been doing its party piece at airshows; using reverse thrust to reverse back down the runway. I see that the three pictures that Kestrel909 has linked to show the same.
Is this significant? That this happens whilst the a/c is actually travelling backwards? I guess this would increase the pressure differential between 'high' and now 'even lower', but would the 20kts or so make that much difference?