These subway-advertisement defacements would probably steam the crabapples of marketing execs more than usual, as they often suggest that the products themselves are crap. They're also highly likely to draw eyeballs, what with looking like annoying computer-error messages.
The hands behind this subversive graffiti campaign belong to Jilly Ballistic, a pseudonym for what Jilly Ballistic calls "New York City's most well-known unknown street and subway artist." I'm not sure how long she has been doing this digital-inspired trompe l'oeil, but examples on her Flickr page date back to at least February. The urban artist also does strange little people in the style of Banksy, sometimes fighting giant pink jellyfish, and has accomplished at least one rude alteration of a sign for Oliver Stone's Savages.
Among other amusing "errors" Jilly has inserted into ads is a notification that, "The following advertisement force quit unexpectedly. Your mind, money and time will no longer be affected." That was on a billboard for Fox's Fringe. Another on a Coors beer sign warns, "We're sorry, the product you are looking for is currently under maintenance due to a lack of quality." Then there's the low-battery warning at Seventh Avenue and 23rd Street, which seems designed to spread vague unease among iPhone obsessives.
With permission of the artist, here are a few of Jilly's recent masterstrokes of street art: