“Upon review at the league office, we have found that a shot taken by Dallas' Monta Ellis with 16.0 seconds remaining in overtime was on the way down when initially contacted and ruled a block by Golden State's Jermaine O'Neal, and should have been ruled a goaltend. The exact trajectory of the ball when touched was impossible to ascertain with the naked eye, and the play was not reviewable.”
Here's video of the play and the ensuing Steph Curry jumper that makes the whole thing worse.
The issue here is that if the officials are going to use as many reviews as they do in a given game, they should have given themselves an out here. Call the goaltend, review it, make sure it's right, then go back and confirm if it is or not. That's the way to handle it.
But sadly for the Mavs, nothing to be done about it now. If they wind up missing the playoffs by a game, this is going to be pretty rough.