Newbie Question About Error Coins

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Errors can happen at the mint. Anything that can happen to a coin later is generally called post mint damage ( PMD). If you can imagine starting with an undamaged coin and making it look unusual, that's probably exactly what happened.

Here's a general guide to common finds, all of which are considered Post Mint Damage.

discoloration - stains from coffee, environmental damage from being buried, etc.
scrapes over much of the coin - damage from sliding on pavement, a parking lot coin
coin not round or bent - one bashed with a hammer
coin smooth on one side - someone sanded it down
mirrored lettering - a vise job, a coin squeezed against another in a vise
rough surface - acid bath
smooth rims, smaller diameter - a coin that has been stuck rollling inside a dryer, a Dryer Coin
clear mounds on coin - glue that has dried transparently
bumps/bubbles - coin exposed to high heat, such as in a campfire
shapes, usually letters or numbers, not raised on coin - Pareidolia
a circular scrape just inside the rim - " Ring of Death" caused by a coin rolling machine