Before the Red Bulls lost the series, however, they lost star striker Bradley Wright-Phillips for the second leg thanks to this yellow card:
That is called "interfering with the goalkeeper's release of the ball" or encroachment. The yellow card was harsh, but it is a foul. It's part of IFAB's Laws of the Game:
A goalkeeper cannot be challenged by an opponent when in control of the ball with the hands
And, as pointed out by The Bent Musket's refereeing expert Jake Catanese, it's not a law that is unfamiliar to US Soccer officials. Jake unearthed a 2010 article from US Soccer on the subject of an incorrectly allowed goal in MLS.
During the course of this interference, Seitz dropped the ball and Moreno shot the ball into the net. These actions by Moreno constituted a violation of Law 12. The goal should not have been allowed and an indirect free kick should have been given where Moreno interfered.
Plata scores! This is a very strange goal. 2-1 Seattle! pic.twitter.com/vVkZMqqNO1— Total MLS (@TotalMLS) August 15, 2016
C'mon, MLS. You're just inventing reasons to fast-track video replays now, aren't you?
RSL's ill-gotten goal didn't affect the outcome of the game. Seattle held on for a 2-1 win. But it shouldn't have been that close (and not just because Clint Dempsey missed a penalty).
Video Assistant Referees are being trialed by IFAB at the moment, and the trial has just been advanced to use in live, competitive professional games. The New York Red Bulls II are the guinea pigs: they hosted the first match to feature a VAR on Friday, August 12.
And it took MLS just two days to come up with (another) reminder of why it is needed.
As it happens, the first VAR in pro soccer history was Allen Chapman: the man who issued the yellow card to BWP back in 2014. Fair to assume he would have intervened to try to prevent RSL's goal being allowed to stand.
IFAB's stated goal for the VAR project is "to eliminate clear errors in match-changing situations". And it will be hard to find a better partner for testing the technology than MLS: a league now seemingly committed to demonstrating the need for VAR on a weekly basis.