It is just seven months and one day since Michael Laudrup's side lifted the first major honour in their 101-year history with a thumping 5-0 victory over Bradford at Wembley.
But the holders were brought crashing back down to earth by another side who also share fond recent memories of the competition - 2011 winners Birmingham.
It was a classic game of two halves in the third-round tie as the Swans saw plenty of the ball in the opening period and hit the woodwork twice within a matter of seconds through Wilfried Bony and Alejandro Pozuelo.
But the Sky Bet Championship hosts appeared a different animal after the break and Dan Burn headed home his first goal for the club in the 56th minute before Matt Green added a second from close range soon after.
Inspired by winger Chris Burke, who played his part in all three goals, Tom Adeyemi also got in on the act with his maiden goal late on - with Bony netting a consolation with virtually the last touch.
Laudrup had made no secret of his desire to lift the League Cup in successive seasons, despite the heavy demands of the Europa League, although he did make 10 changes from Sunday's Barclays Premier League victory at Crystal Palace, with midfielder Jonjo Shelvey the only survivor.
Yet his in-form side, having won three of their last unbeaten four including a memorable 3-0 triumph in Valencia, were unable to reproduce their heroics from the Mestalla in the midlands.
The only downside of the evening for Birmingham was that there was only 7,470 in attendance to watch what was arguably the biggest upset of the third round.