Barrel Distortion Example

The straight lines of the window bottoms in this picture show the barrel distortion present in this 8mm lens. Another image quality-related issue with lens distortion is framing the image level in the viewfinder. The built-in level in the 60D definitely helps.
 
Distortion can be corrected in postprocessing, but this is a destructive process. I should mention that barrel distortion is common in wide angle zoom lenses at their widest focal length.