The world's first adhesive postage stamp was the "Penny Black," issued in Great Britain in 1840. After it had been in use for a while, officials noted that cancellations with black ink were difficult to discern on the stamps (thus risking their being reused by the public). Some cancellations were done with red ink, but the simpler expedient was to change the design of the postage stamp to red, and cancel them with black markings.
The "Penny Red" served as postage for a first-class letter in Great Britain for about 40 years (which says something about inflation and economics of the era). These stamps were therefore printed literally by the billions. Most therefore are today of only minimal monetary value. Except for the one shown above.
When you print something by the billions and billiions, the metal plates used to print them wear out, develop cracks and broken highlights, or generally become dull. New plates are then created. In nineteenth-century Britain, the printing plates were sequentially numbered, and many stamps of the period carry "plate numbers" placed modestly somewhere in the design. I have highlighted with yellow ovals in the embedded image the position of plate numbers within the lacework on the lateral borders of the Penny Red.
Plate #77 was found to be defective and not formally used. The unused copy shown above is one of only nine copies known to exist; it resides in the archives of the British Library and carries a catalogue value of approximately $175,000 USD.
If you have some Penny Reds sitting in an old album in a closet, you can look up the value of the plate number in a variety of philatelic catalogues. Sound used copies are valued in the range of about $3-20 USD, with mint copies approximately a log power higher.