I stand up, take a step from the desk, but a mix of cables was tangled around my leg, and pulled hard on my AKG K601 headphones. The left speaker stopped making a sound. This particular headphone model, which I absolutely love, is discontinued, but no worries. I suspected a broken soldering joint, which would be easy to fix. As usual, the hard part is opening the sucker up to get at the actual problem.
Turns out you have to rotate the grill over the rear of the driver a little bit in anti-clockwise direction. I used paper clips wire to get a good grip, but tiny wood screws would also work. There is also a service manual with some advice how to proceed.