In June 2013, I noticed a very odd little piece of self-referential art at the Nostrand Avenue A train stop in Brooklyn. It was a photograph of one of the mosaics that decorate the station walls at intervals, declaring the station name: NOSTRAND AVE. And it was glued to the tiled wall directly below one of the mosaics. In a sense it was like seeing a mirror reflected in another mirror.
I never did find out the origins of the photo of the Nostrand Avenue sign next to the same sign. A few days ago, a similar 'work' appeared at 23rd Street, on the uptown A, C and E lines: