NGC 1977-S proof Jefferson nickel on cent planchet. The coin has the design of a nickel but is struck on the planchet of a cent. The coin has deep mirrored fields and frosted devices, and contrast with the copper planchet beautifully.
The error occurred when a cent planchet was fed into a press that was striking nickels and was struck, creating a proof nickel that was struck on a cent planchet instead of a normal nickel planchet.
Errors are rare on proof nickels because of the strict quality controls and because it is tough for them to get out of the Mint because the proof sets they are housed in pretty much only allow coins to be their normal size (or smaller.) Because this coin is smaller than a normal nickel, errors such as this could conceivably come out in a proof set.
Many rarities such as most U.S. patterns, the 1913 nickels, and more have made it out of the Mint through one means or another, and all are highly valued by the collecting community today. Some proof errors came out when found in a safe deposit box in San Franscisco. Still others came out through other unknown means. There are a number of possibilities with errors such as this.
PF66 Red/Brown with almost entirely red surfaces. We do not see how this coin did not get a "red" designation!