WILLIAM SALIBA was ineligible to take a penalty in Arsenal's FA Cup defeat to Manchester United due to a little-known rule.
The Gunners at the Emirates on Sunday following a 1-1 draw.
Mikel Arteta's men dominated the match but were unable to find a winner - despite having an extra man due to Diogo Dalot's second-half red card.
And Dalot's dismissal resulted in Saliba not being able to take a spot kick.
FA Cup rules prohibits a team from having more available penalty takers than their opponents in a shootout.
That meant one of the Gunners had to choose one player to step aside ahead of the shootout, with France international Saliba the sidelined star.
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United ran out 5-3 winners in the shootout, with striker Joshua Zirkzee scoring the decisive penalty.
Kai Havertz missed the Gunners' second penalty after Martin Odegaard made amends for his own spotkick miss in normal time.
Havertz's failure to convert from 12 yards added to a catalogue of earlier misses.
And sick trolls ended up targeting the Germany international's wife, Sophia, with vile messages after the final whistle.
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Arsenal ratings vs Man Utd as Martinelli looks lively but wasteful Havertz costs Gunners another trophy chance
KAI HAVERTZ missed two sitters and a penalty in the shoot-out as Arsenal were dumped out of the FA Cup.
The Gunners huffed and puffed but failed to find a winner, even after extra-time, against Manchester United’s 10 men.
And it all went wrong in the penalty shootout as Altay Bayindir saved Havertz’s effort.
Here is how SunSport's Kealan Hughes rated the Arsenal stars...
DAVID RAYA - 6
Could do nothing about United's goal and produced a stellar stop to deny Zirkzee in extra-time.
JURRIEN TIMBER - 7
Very effective on the overlap and worked well being fed by Odegaard.
Tested a couple of times by Alejandro Garnacho who proved a problem on the counter-attack.
WILLIAM SALIBA - 7
Dealt with Hojlund well although the striker did not pose much of a threat.
GABRIEL - 6
Terrible error to lose possession to Alejandro Garnacho who was put through on goal and set up Bruno Fernandes to score.
Made amends with a brilliant finish Bukayo Saka would have been proud of.
MYLES LEWIS-SKELLY - 5
A quiet game at left-back, probably should have got forward more particularly after United went down to ten men and had Garnacho filling in at right-back.
JORGINHO - 5
Kept things ticking over in the middle and was always an outlet for his team-mates.
MIKEL MERINO - 4
Anonymous in the middle. Difficult to see what he brings to this Arsenal side.
Missed a good chance to put his side ahead moments after the equaliser and was taken off shortly after that.
MARTIN ODEGAARD - 7
A pleasure to watch. Made everything happen, his ability to turn on the ball in the middle of the park created opportunities time after time.
But he missed the chance to win the game from the penalty spot.
GABRIEL JESUS - 6
Lovely pass to break the lines and put through Martinelli, who found the back of the net but was offside.
Fell awkwardly when defending a cross in the first half and was carried off on a stretcher.
GABRIEL MARTINELLI - 7
Was the main attacking threat in the first half with his direct style causing the visitors all sorts of problems.
Produced good movement and a finish to match but his goal was disallowed for offside.
KAI HAVERTZ - 3
Won the penalty after a deft touch past Maguire.
But he missed two sitters including one just three minutes from time to win the match, and his penalty miss proved decisive in the shoot-out.
SUBSTITUTES
Raheem Sterling (for Jesus, 40mins) - 6
Looked sharp when he got the ball and did well in the build-up to Havertz winning the penalty.
Declan Rice (for Merino, 73mins) - 6
Almost scored with what might have been his first touch but his header was brilliantly saved.
Should have grabbed the winner in the dying moments but his low effort was again superbly matched by Bayindir.
Leandro Trossard (for Lewis-Skelly, extra-time) - 5
Will wonder how he didn't score after Matthijs de Ligt produced a goalline clearance to deny him.
Blazed over with an effort from just outside the box.
Thomas Partey (for Timber, 101mins) - 5
Put in a few good crosses. Came close to a winner with an effort that was deflected narrowly over the bar.
Kieran Tierney (for Jorginho, 113mins) -
His arrival onto the pitch was greeted with a big cheer but he did not have much time to impact the match.
Victory in London has earned United a fourth-round clash with Leicester City, who they'll welcome to Old Trafford on February 8.
Red Devils boss Ruben Amorim said: "We deserved to win, Arsenal didn't deserve to lose but we deserved to win.
"The way we played, the way we fought was really tough since the beginning.
"We didn't have the ball too much but we had control without the ball.
"I want to also say thank you to all the fans. It was unbelievable.
"I did not understand that all that area was our supporters, so it was a good day for us now we have to rest the players, I'm already thinking about the next game they're really tired.
"Sometimes we have the feeling that everything in the end is going to be okay.
"I felt that since the beginning of the second half, even after the penalty. I think the Arsenal team felt that maybe today is not their day.
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"I'm really pleased for the way they controlled the game without the ball. We defended very well, set pieces we were really strong.
"It was a good day but now I'm thinking about the next game and worried about the next game."