Bach's crab canon from "The Musical Offering." This is recorded so that the audio can be played backwards (just like the Beatles used to do!) and still sound exactly the same. It's played twice through, and the second performance is just a reversal of the audio waveforms from the first performance. In fact, the original recording was of a single synth organ playing the top part - that was then audio-reversed and added to the original to create the first performance you hear.
The bottom, lightly shaded staff, shows what the second voice sounds like, but it's more fun to try to watch just the top staff for both parts. Just follow the crustaceans.