If you happen to own an MFJ-260B dummy load and it’s giving you weird SWR values, take the cover off and roll the power resistor in its mounting clips:
My buddy Aitch discovered that oxide / corrosion / dirt buildup between the resistor and the clips can produce absolutely baffling results, even while passing enough current to warm up the element, far more power than you’d think would burn away any crud.
The Smell of Molten Projects in the Morning 