You've spent the best part of an hour trying to set the brakepads of your V-brakes or Cantilevers, but no matter what you do, either the left or the right pad touches the rim while no pressure is applied to the brake lever. You are silently going crazy.
Well, the rim might be out of true! If the only thing that fixes the problem is widening the brakepads, then that's a clear indication that this is indeed the case. Of course, if you widen the brakepads too much, you will have no braking power, so the only medicine is to true the wheel.