That's a very common area for die chips. As those dies aged, the entire upper portion of the 9 would often become filled.
The die is negative or reverse from the coin. So everything you see raised on a coin is cut into the die or recessed. So the 9 is cut into the die which means the area inside the loop of the 9 no longer has any support. Just like digging a circle in dirt. The area inside the circle will collapse if you step on it. So, in the die it breaks off and fills with planchet metal when the coin is being struck-"die fill." Since it's a hole, it shows up as the opposite or raised on the coin. 1958 Lincolns are notorious for die failure.
Good answers from longnine009 and jallengomez. Die chips are also common on these cents dated 1957-D: On many dates you can find die chips between the B and E of LIBERTY (commonly called BIE errors):
