The Universal Earphones Project, designed by the science-wizards at the Japan Science and Technology Agency, has come up with a pair of headphones that know which ear they're sitting in. So you need never get your stereo mix in a muddle by sticking the left earphone in your right ear.
Ahaha, I see what you did there. One earpiece sports a proximity sensor; if it's pointing towards you it plays the left channel, if it's pointing away it plays the right channel. It can even work out if you're sharing your tunes with a friend and play a mono mix to both headphones. Check out the video below for more details.