I have no argument that the call on the play in question was ticky tack at best and probably should not have been called.
HOWEVER...
The play before that Henderson was totally mugged while trying to run his route and no flags were thrown.
It was an awful non call and would have resulted in exactly the same situation of a first down, just one play earlier.
The real problem isn't that there was a flag thrown on the play to Kupp. It was that they threw a flag on THAT play after NOT throwing one on a worse foul the play before.
Also, one of those 4 accepted flags in the last 2 minutes was against the Rams and wiped out a TD.
All that said, the Rams scored again AND Joe Cool III had the ball again needing only a FG to tie and couldn't do it.
One last observation on the end of game officiating...
The Rams were inside the 10 yard line for that period of the game. That means the officials were also all writhing that small area which gave them a greater view of everything going on. They all had eyes on a much smaller segment of the field so if there were fouls being committed they would be more visible to the crew. It's no surprise they picked up in what a lot of us saw throughout the game down there inside the 10.
Thus, I have ZERO angst about the officiating near the end of the game.
