Strange "M" on quarter:

Bluebootedcats

I recieved this quarter as change at the store last week, I do collect U.S. state quarters and other odd/misprint currency, but I'm not sure what this "M" could mean, any thoughts?

silverpop

that was added after the coin left the mint it's called PMD
as for the M could be someone who was bored and carved it into the coin

Aspie_Rocco

It is a counterstamp or stamp, done by someone after it left the US mint. Post mint damage or pmd is what we generally call it. Someone stamped an M or W into Georges head.
It could be any thing; initial, part of a game set, an act of boredom, part of a secret organizations plan to do something nefarious, a piece of a word puzzle...