That's called a "BIE" Die Chip or Break, whatever you want to call it, but anyways, it happens when a small piece of metal breaks away from the die, causing a raised "blob" of metal to appear on freshly struck coins. Cuds-on-coins.com does catalog these if you want to take a look, and see if yours is listed.
http://cuds-on-coins.com/ldb4-1957-2/
Errers and Varietys.