If you own an old Gold Top Les Paul you are no doubt painfully aware of how noisy the pickups are. If you are brave enough to take one of the pickups apart it is possible to make the guitar hum-bucking when using both pickups together. Remove the cover from one of the pickups and reverse the direction of the bar magnet within. You also need to reverse the polarity of the pickup by inverting the hot and ground wires.
After doing this, you can check magnetic polarity of your pickups by means of another magnet. Bring the magnet over each pickup. It should be attracted by one and repelled by the other.
Be warned that this is a dangerous operation and do not attempt to do this if you don't feel qualified.