After it left the mint. It's not an error. Some type of stain, oil, chemical, acid, environmental damage, etc.....pick one. Use your imagination. Whatever you come up with, works for me. Spend it before it multiplies!
42 years in circulation. We can only speculate. But if you evaluate the substance on the coin, then you can determine potentially how it got there. For instance, let's say you find out that it's dried up soda pop (pepsi, cocacola, dr pepper, what ever ..). Then you can speculate that it was in a cup holder and someone spilled it on the coin and it then dried before or after it was used to buy something and put back in circulation.
PMD. Post mint damage caused by ?. As @Clawcoins said, looks like a cup holder coin that got stained from spills.
@Mountain Man did you just “coin” a new phrase?? Cup holder coin!! Op you take one for the team and lick the coin. My vote coca-Cola!