Obafemi Martins scored in 89th minute winner after a blunder by goalkeeper Wojciech Szczesny and Laurent Koscielny, giving Birmingham a 2-1 victory over Arsenal in the League Cup final for its first trophy since 1963.
Szczesny and Koscielny both went to stop Nikola Zigic's header in the penalty area and the ball landed at the feet of Martin, who slid it into the net.
"I think it was my second touch," said Martins, who joined Birmingham on a six-month loan from the Russian club Rubin Kazan last month. "I think this is the easiest goal I've ever scored in my career."
Zigic, at 6-foot-8 the Premier League's tallest player, had put Birmingham in front with a header in the 28th minute. Robin van Persie of Arsenal evened it 11 minutes later with a volley.
Birmingham won the 1963 League Cup final. Its victory Sunday left Arsenal searching for its first trophy since the 2005 FA Cup.
Birmingham is battling relegation from the Premier League, two points above the drop zone. Arsenal is second behind Manchester United.
"It's an achievement when you consider who we were up against," Birmingham manager Alex McLeish said. "We weren't given a prayer by the pundits and the bookies but we believed today. It was a titanic performance by the players."
Arsenal has lost three finals since 2005. That lack of success, manager Arsene Wenger said before the match, had placed "a weight on the team." Wenger hoped the final would show how far his club has matured, but Arsenal was sluggish and struggled to impose itself.