Pitts special, Radio controlled aircraft Mid Air
 

and it isn't a small one at that............

YouTube - Biplane and RC Airplane Midair Collision

longer version with aftermath...

YouTube - Real Biplane crashes into a 46% R/C plane.MOV

Nasty. Could have been much worse.

From what I've been able to gather, from various forums, it seems most likely to either have been a breakdown in communication at a badly-organised (and possibly unauthorised) 'airshow', or a straightforward act of foolhardiness by the Pitts pilot - making a low smoke-on pass without authorisation.

There are suggestions that the Pitts pilot has already 'apologised' for the incident - if so, I suspect he didn't consult a lawyer first.

One thing worth noting is that regardless of whether flying an RC aircraft on a runway during a display is sensible or not, making a low pass with people on the runway isn't. If you look at the video, it is apparent that the collision could have almost as easily occurred between the plane and one of the RC demonstrators.

The aircraft involved wasn't a Pitts Special, it was a SA-750 kit plane. here is the accident record on the FAA website (record #2). The RC plane was a 46% scale "AJ Slick" aerobatic / 3D model from the 3D Hobbyshop range of model planes.

IMHO the pilot of the SA-750 was lucky to only impact the balsa / lite ply structure of the model plane as a direct hit on the 150cc petrol engine would doubtless have caused enough structural damage for the outcome to be far worse.