J.R. Smith is a known knucklehead. Look no further than his ill-advised shot attempt in the New York Knicks' loss to the Houston Rockets on Friday night. Smith was at it again Sunday against the Dallas Mavericks, although this stunt didn't actually hurt his team.
Right as Dirk Nowitzki was attempting a free throw, Smith untied one of Shawn Marion's shoes (via @BeyondTheBuzzer):
Very, very sneaky.