Rotated Die (Error)

A Rotated Die is a Mint Error when the die of the either the Obverse or Reverse is rotated.

What Happens
A US coin is purposly rotated the way it is so you can flip it over and the side you flipped to is right side up. Sometimes the dies that strike either Obverse or Reverse can be tilted more to a side.

You can't tell if the Reverse or the Obverse is rotated. It can be

Coins with at most 8 degrees are considered in the margin of the US Mint. 90 degrees and over are highly valuable and collectible. 15 degrees and under is not very valuable.

Resources
Lincoln Cent Resource (Information)

eBay.com (Image)