Back in the days when television was way more open to experiments, the artpiece "self burial" by Keith Arnatt was shown at WDR 3 programm without any announcement
Between 11th and 18th of october 1969 each evening there was shown one of eight photographies of the artist where you see him gradually sinking into the ground. The pictures where shown for several seconds right after the main news broadcast at 8.15 pm and for another time one hour later in the middle of whatever program was running.
The mystery was cleared at the end of the week by an interview with the artist.
Also nice: "Notes from Jo 1990-1994", a series of blown-up images of the notes left around the house by his wife shortly before she died.