A professional foul that had more than just Gus Poyet scratching his head.
Consider Brown's dilemma. A shocking, sloppy back pass from Phil Bardsley put Hull's Shane Long through on goal.
At such an early stage of the game, did he think he would get the benefit of the doubt from the referee if he chopped Long down?
He could have made a genuine tackle, although Long would probably have skipped it.
Or Brown could have let him go and let Long take his chances against Vito Mannone.
The consensus around me at the Stadium of Light was that ANYTHING would have been better than the one he made.
He didn't give away a penalty but that was a touch-and-go decision that was 2" either way.
But to instantly end his involvement in the game was nothing short of stupid!
Same old Sunderland - we get a great run going then we shoot ourselves in the foot.
Hull failed to score from the resulting free kick on the edge of the penalty area but the whole game changed and we were never going to get anything out of it.
With Fabio Borini withdrawn to enable Poyet to make his post-red card adjustments, the best Sunderland could manage in the first half was a stunning Adam Johnson volley.
A rare period of bright Sunderland play in the second half was rudely interrupted by a heavily deflected cross-shot that fell, cruelly for Sunderland, onto Nikica Jelavic's head.
The game was then beyond the reach of a home side unable to muster a serious threat to the Hull goal.
Steve Bruce got his tactics right, and the three former Sunderland men on show all produced "told you so" performances.
With Borini gone, Elmohamady combined well with Liam Rosenior to torment the exposed Marcos Alonso on the right.
When Sunderland played at Hull in November, Lee Cattermole and Andre Dossena were both shown straight reds before half-time.
Since that 1-0 defeat, Brown has been utterly crucial to Sunderland's run of better results, and he will be missed.
Brown has now seen red three times this season, though one was later rescinded.
Gus Poyet: "We can't look back too much on this game because it's not a normal game when you play 85 minutes with 10 men."