Robbo was a graffiti artist who had one piece remain essentially untouched for over 25 years – the equivalent of two Ice Ages in street-art terms. Robbo was one of the most respected artists of all time on the London street scene. Over the years his piece had been covered up, so Banksy painted over it. Robbo took it as a diss and started a little tiff. The two of them "battled" back and forth on the same piece of wall for a couple of years, but when Robbo had an accident, Banksy tried to recreate the original. You can see the battles on Banksy's website here. Just look for the question about Robbo.