The Mavs had the ball in a tie game in the final minute, and Monta Ellis drove to the basket and attempted a potential go-ahead floater. Warriors big man Jermaine O'Neal snatched the ball out of the air, only it appeared awfully close to a goaltend (via @cjzero):
No goaltending call was made, and because there was no call made on the floor, the play wasn't eligible to be reviewed. Curry then delivered the dagger on the other end, dropping the Mavs back into ninth in the Western Conference playoff race.
Afterward, Dirk Nowitzki sought an explanation from the officiating crew of Danny Crawford, Sean Corbin and Eric Dalen. Crawford was the official closest to the play and actually has some history with the Mavs, and Nowitzki clearly wasn't satisfied with the explanation he received from the longtime ref, according to Tim MacMahon of ESPN Dallas:
"I think his layup has a chance to get to the rim, and if that's the case, you can't just get it out of the air," Nowitzki said. "To me, that's a goaltend. I asked the referees what happened. The explanation was that the ball was two feet short. If that's the case, then he can get it out of the air, but where I was from, I think it had a chance to at least hit the rim. That's a goaltend to me."
Mavs owner Mark Cuban, who's known for criticizing officials, made it known to the crew he was unhappy with the non-call. However, Cuban declined to comment after the game.
Ellis also didn't comment, leaving the locker room before the media was allowed in.
O'Neal naturally defended himself, saying the block was "a second away from goaltending."