Rotated Dies
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.  
The following is a list of known Lincoln Cent Rotated Die Errors  (rotated 90 degress or more)
from www.rotateddies.com/

     1910                90 degrees       
    1920                110 degrees   
    1924-D             120 degrees          
    1924-D             180 degrees        
    1936-D             95 degrees        
    1940-S             129 degrees        
    1966                135 degrees         
    1972-D             90 degrees         
    1972-D             100 degrees          
    1972-D             120 degrees         
    1972-D             140 degrees          
    1972-D             150 degrees          
    1973-D             90 degrees         
    1973-D             100 degrees        
    1973-D             105 degrees       
    1973-D             120 degrees         
    1973-D             135 degrees        
    1973-D             170 degrees        
    1993-D             90 degrees          
    1993-D             180 degrees          
    1994                 165 degrees  

If you are interested in figuring out how much your coin is rotated, you can build a "Coin Degree
Finder" by clicking here.