The New York Times reports:
"Workers for Transport of London, the government body responsible for the city’s transport systems, have painted over a mural by Banksy, the celebrated graffiti artist, BBC News reported yesterday. The work, right, near the Old Street underground station, was taken from the film 'Pulp Fiction' but showed the actors Samuel L. Jackson and John Travolta clutching bananas rather than guns. It was one of Banksy’s best-known efforts, estimated by some to be worth [$600,000]. .... In March most of a Banksy mural in East London was stolen, and an early work was mistakenly painted over in the city of Bristol by graffiti-removal contractors. In February two smaller works in London were also painted over."
A spokesman for the agency says their "graffiti-removal teams are staffed by professional cleaners, not professional art critics." (Norm Geras wonders if that's a slur on the staff.)
You can see an image of the work with the BBC story here. Previous mentions of Banksy here and here.