Watch: Mavericks get sloppy to end first quarter

Dallas had a six-point lead and the ball late in the first quarter, but with 37 seconds left, Luka Doncic missed a step-back jumper and stopped to complain to the referees that he was fouled. Boston didn't hesitate, and with a man advantage, Sam Hauser drilled an open three-pointer.

On the next play, Dallas went to Doncic again, for another long step back. This time, Doncic hit the floor, and Boston's Jayson Tatum leaked out. While it wasn't Doncic's responsibility to get back, the chaos of the missed shot allowed Boston to get another easy bucket and finish the quarter down only a point.  

When they got the ball back with 48 seconds to go in the quarter, Dallas had a chance to essentially guarantee it'd get two shots to Boston's one. But Doncic took a shot early in the shot clock to give Boston a chance at a two-for-one opportunity with 34 seconds left. Then his second shot came with eight seconds left, giving the Celtics another chance.

After a quarter where the Mavericks led by as many as 13 points, their sloppiness left them with only a one-point lead. Those are mistakes a team down 2-0 in the NBA Finals simply can't afford to make.