How Did the Three Legged Buffalo Nickel Occur?

By GovMint :

The famous "Three-Legged Buffalo" Nickel, minted in 1937, resulted from a die error at the Denver Mint. This intriguing mistake occurred after a die clash, which means the obverse and reverse dies collided without a planchet, ultimately causing damage to the dies. Excessive polishing was used to try to fix the dies and remove the clash marks, however over-polishing inadvertently eliminated part of the buffalo's front right leg, leaving only a hoof and creating the illusion of a three-legged buffalo.

Sources Used:

https://www.pcgs.com/news/1937-d-3-legged-buffalo-nickel-capturing-attention