The image to the left is an example of a 1994 cent whose dies have been rotated 165 degrees.
In a normal U.S. coin the obverse and reverse are struck in such a manner that if you are looking at the obverse image right side up and flip the coin over the reverse image will be right side up.
Classifications of rotated dies can be as high as 180 degrees. Since it is unknown which die, the obverse, reverse, or combination of both, was out of place, the error is referred to as a "rotated dies" mint error.
The U.S. Mint allows for an error margin of less than 8 degrees. Rotated die errors are usually not collectible unless they are at least 15 degrees. Rotated die errors are most valuable after 90 degrees with 180 degrees being the most valuable.