In the 89th minute of the encounter, with the score 1-1, a routine long ball came bouncing in towards the Arsenal keeper Wojciech Szczesny. Defender Laurent Koscielny looked like he was going to play the ball, but left it for the charging Szczesny—who fumbled it into the path of Birmingham substitute Obafemi Martins.
The striker made no mistake in tapping the ball into the empty net from ten yards out, launching a raucous celebration from the Birmingham faithful.
Arsenal was left shattered, and stoppage time passed without the Gunners mounting a threat. As Birmingham celebrated a hard-earned victory, Arsenal players dropped to the ground in despair.
The shock goal from the mix-up came against the run of play, as Arsenal had ratcheted up their attack in search for the winner. Birmingham looked pushed to the brink, but were determined to fight.
Centre back Roger Johnson was limping noticeably, and it appeared as if the Gunners' pace had finally broken the underdogs at Wembley; they were struggling to keep up.
Birmingham, however, had played a determined game throughout. They were unlucky not to be awarded a penalty after only two minutes—when the linesman incorrectly ruled Lee Bowyer offside. Before play was stopped, in the process of rounding Szczesny he was clearly brought down illegally.
Nevertheless, the Blues opened the scoring in the 28th minute when the tall and powerful Nikola Zigic headed home from short-range off a set-piece.
Arsenal responded quickly, however, with a great play from Andrei Arshavin to get to the byline and then cross it back to Robin Van Persie, who peeled off his man to produce a wonderful finish—falling away from goal.
The teams went to the half tied 1-1, and it was Birmingham who came closest to scoring after the break.
After a blocked shot from distance, the ball fell to Keith Fahey who produced a rocket from 20 yards out that had Szczesny beat, but the ball clamored off the inside of the post, and was eventually cleared by the exposed Arsenal defense.
The Gunners went on the attack as full-time neared, with Birmingham keeper Ben Foster put to the test.
On a counter-attack, Samir Nasri's shot was saved by the England international; before he produced fine stops on shots from Niklas Bendtner, and then Nasri again.
With Arsenal enjoying the most consistent possession they'd had all game, they were gutted by the final goal.
Foster was named Man of the Match for his vital role in keeping the match 1-1, while his counterpart, Szczesny, will be remembered in infamy.
The win was a special one for the Birmingham side, as they had shown a tremendous amount of grit. Their fans were in jubilation, a celebration only surpassed by that of the men in blue on the pitch.
It would be harsh on Arsenal to dismiss their performance as complacent.
They were set on winning this trophy for the club and their fans, and it showed in their tempo. The Gunners were playing without their superstar and captain, Cesc Fabregas; as well as injured winger Theo Walcott, two vital components to the Arsenal attack.
Fabregas's absence was most notable in the attacking third for Arsenal, where through large parts of the game they lacked the creativity to break down Birmingham's stubborn defense.
This has been the case all season for the Gunners when Fabregas isn't on the field, and it underscores what a brilliant player he is. His replacement, Czech international Tomas Rosicky had a poor performance, as he had a couple of great chances to score that he squandered. He also lacked the right chemistry with the in-form Robin Van Persie, whom Fabregas has made an art of supplying throughout their tenure at Arsenal together.
The blow will be hard to stomach for Arsenal and their fans because the six-year trophy drought seemed destined to end today at Wembley. The crushing manner in which they lost the game has the potential to wreak havoc on the minds of the young squad.
Szczesny, only 20, will remember this blunder for the rest of his career.
He should be consoled by the fact that he has generally played very well this season, and seems poised to have a fine career for Arsenal.
On the upside, Arsenal are still in good position to win three other trophies this year. The keys will be staying healthy and overcoming this psychological setback.
If they can turn the sting of defeat into a greater will to achieve, then there's no reason they can't catch a less-than-convincing Manchester United side atop the table. But they must do so quickly, as a very pivotal two weeks lie ahead—with potential cup clashes away to both Barcelona and Manchester United in the Champions League and FA Cup, respectively if Arsenal can first get by Leyton Orient on Wednesday.
Birmingham deserved their upset win today, and while it looks like this Arsenal side still may prosper, today was simply not their day.