That's a clipped planchet error. Nice coin. You can find out more about clips and how to spot them by reading this link and searching for the links to clipped planchet errors. http://www.error-ref.com/part-v-planchet-errors/
I never understood why someone would collect these. How would anyone know it's a mint error and not someone clipping these in their garage?
It's all explained here: http://www.error-ref.com/part-v-planchet-errors/ There are telltale signs such as a weakness directly opposite (on the rim), a tapering effect on the clip, flow lines around the area, etc.
