When heavy mechanical grease gets packed into the coin stamp (the die), it acts like a cushion. When the coin is minted:It flattens out the details, turning normally rounded or curved shapes into boxy, faint, or blocky squares.It prevents the lettering from striking fully, making letters—like the "R" in AMERICA on the reverse—look very light, faded, or partially missing.Because the "5" in your 1965 date looks a bit blocky and squarish, and letters on the back are lighter than normal, your penny may very well have a minor grease-filled die error. These are exciting pieces of minting history to find in pocket change!