Clashed Dies
A die clash is when the pair of dies slams together without a planchet between them. The result is leftover marks where the design from one (or both) die transfers partially to the other.

In the photo the light source is coming from the top (to help you figure out what's sunken and what's standing out). Notice near the top of the photo that you can see an image of the steps (remember this is upside down) and the bottom of the memorial building. You can also see how this mark blends in and runs underneath the back of Lincoln's head. This is a tell-tale sign that this is a real die clash.

This particular example was hit with a hammer, I can tell by the shape of the stress marks in the copper plating. The two most important things to notice here are that the letters of LIBERTY are affecting the highest points of the design on the reverse (which would be sunken-in on the die, thus not affected), and that the position of the word is exactly opposite where it should be. Get out a penny, look at the word LIBERTY, and follow it to the reverse. You will see that if this were a clash, it would be on the left side of the reverse, not the right side, as is the case here.
Another important feature is to notice that the designer's initials are flattened inside the "B" of LIBERTY. This indicates a normal strike, then tampering thereafter. Impossible with a die clash.