I played a couple of movements once from a piece called "The Thousand Symphonies" of Dick Higgins. The composer set up a bunch of blank orchestral staff paper a distance away and fired a few rounds into them. The paper was photocopied and bullet holes would be interpreted one way, wrinkles and dark blotches on the page, etc. The page would be given a duration during which one would read the assigned staff right to left and/or left to right, and the player would be allowed to interpret these as they wished generally.
I have to say it actually turned out to be a quite powerful and awesome concert. This music can actually be pretty effective and even enjoyable for the audience if done well.
By the way, the encore piece was a simple instruction:
Play a sound that is as exhausting as possible. Continue to play this sound until you cannot play it anymore.
I opted to play two trumpets at once and screech with my voice while playing these. Sustained a slight injury, actually.
The conductor (a guy in his 70s attired in a burgundy velvet sweatsuit) was genius, he grabbed a hold of his conductor stand and gradually began shaking the bejesus out of it, finally collapsing on the floor.