Doryphore

Doryphore \DOH-ree-for\ , noun;
1. One who draws attention to the minor errors made by others, especially in a pestering manner

This word is first attested in 1952 from French doryphore ("Colorado beetle") from Greek doryforos ("spear-carrier"). The modern meaning is apparently connected with the British diplomat, writer, and politician Sir Harold Nicolson, but I'm not sure exactly why.

This word made me think of those people who love to point out grammatical mistakes on people's Facebook statuses.