Isn't it accepted that the best graffiti of all time was when someone added an addendum to one of the many "Free Nelson Mandela" posters fly posted all over the country back in the days when he was still in prison.
Some wag had added "with every packet of cornflakes" to one. Most funny.
Haven't got a picture of it, but on the London Underground a few years ago they had a campaign against assaults on staff...the poster showed a picture of prison gates with the strapline "If you assault a member of London Transport staff you could end up here"...to which someone had added "Morden Depot"!