Remember when cash machines first came out? They had opaque perspex barriers that slid down to cover the screen and keyboard when not in use.
This was to prevent vanadlism, but as they were giving cards to everybody, it simply meant that vandals could make a right mess, then hide their handiwork until the next customer came along.
You would put your card into the slot, and the barrier would rise like a theatre curtain. To reveal a 'tableau' of greenies, marker pen and chip-shop Pies smeared all over the interior.
Today, that sort of thing would win the Turner. *Sits back, puffs on pipe, contented that he has had a dig at modern art, but vaguely annoyed he didn't get to mention split-in-half cows.*