1956 Les Paul Junior - This one came in for a cleanup & setup after having spent many years in the case. The strings were nice 'n rusty and the frets were oxidized and covered in crud. A quick plug-in to an amp revealed a very weak output as well as crackly pots & an intermittent jack.
The original pots & jack were noisy, intermittent, & simply in need of some service. After a thorough cleaning there was still an extremely weak output from the guitar. Measuring the resistance of the pickup showed a normal reading of about 8.5k.
Inspection of the pickup revealed that one of the two bar magnets had been flipped around in the wrong direction and was in effect cancelling out the magnetic field, causing the pickup to have a very weak output despite the normal resistance reading. After correcting the orientation of the magnets the guitar came back alive with the sound that only a '50s Junior can deliver. When I explained this to the owner he recalled buying the guitar years ago for a really great deal, and putting it away because "It always sounded kind of weak". Apparently at some point many years ago someone disassembled the pickup and had re-assembled it incorrectly, and figuring that he'd ruined the pickup, sold the guitar off cheap !