John O’Flynn scored an injury-time winner as the Cobblers were beaten in the last gasp of a game against Exeter City in which they were dominant.
Town paid the price for failing to take their chances and building on a vast majority of possession against a Grecians side who secured a 2-1 victory with almost the final play of the game.
Jacob Blyth had put the hosts ahead after only six minutes and even when Scot Bennett cancelled that goal out with a bundled equaliser at a corner, Northampton still looked like they would go on to score again.
But Clive Platt missed two excellent goalscoring chances and - as it appeared they would have to settle for a point instead of all three - O’Flynn headed home to secure an unlikely Exeter victory.
The home side made a very bright start to the game, with Platt continuing where he left off last week by dominating in the air and Chris Hackett seeing plenty of the ball down the right wing.
Although most of their good positions were carved out through technical football, the opening goal had its roots in a more direct style.
A long ball from the defence was flicked on by the head of Roy O’Donovan to Clive Platt, whose path picked out the run of Blyth perfectly.
After steadying himself and running through on goal, Blyth then guided the ball expertly past Artur Krysiak as he advanced to meet the striker just inside the penalty area.
Blyth should have had a second after a quarter of an hour when he found himself presented with the ball with just the goalkeeper to beat after a Platt flick-on had created confusion in the visiting defence.
The finish, though, was somewhat rushed and the Leicester loanee ended up stabbing the ball over Krysiak but wide of the target.
Platt himself then hit the bar with an attempt that he reached at after another Hackett cross had deflected into his general area eight yards from goal.
The big striker was in dominant mood and underlined that with a glanced header from a Kevin Amankwaah cross that landed on the roof of the net moments later.
Exeter, though, fought hard and found a way back into the game with an equaliser that should make painful viewing for the home defence.
A Craig Woodman corner on the right was whipped into the box and Scot Bennett pounced on it from no more than three yards and with the ball barely a yard off the ground to smother home an equaliser.
Platt could - and almost certainly should - have put Town back in front when he had a free header at an excellent Joe Widdowson cross but planted the effort high of the target.
The big striker was later put through by an excellent first-time Matty Harriott pass (which was also his first touch of the game) only for Krysiak to burst off his line and repel the sidefooted attempt.
Danny Emerton enjoyed a neat little 10-minute cameo that included a good cross and a blocked shot as well as a very useful piece of defending in his own final third.
The Cobblers looked like they avoided the cruel finish once, when a moment of hesitation at the back allowed former loanee Sam Parkin a clear run at goal.
Parkin rounded Matt Duke and rolled a shot towards goal that Lee Collins, so often an effective sweeper in this game, cleared off the line.
But the sting came eventually. David Wheeler found space around Widdowson in which to fire in a cross that O’Flynn, arriving at pace and with perfect timing, headed decisively past Duke.
It was a cruel end to a game from which Northampton deserved so much more.
Cobblers: Duke, Amankwaah, Collins, Heath, Widdowson, Hackett, Deegan, Carter, O’Donovan, Platt, Blyth
Subs: Harriott (for Carter 67), Tozer (for Amankwaah 79), Emerton (for O’Donovan 79)
Not used: Morris, Hooper, McNamara, Toney
Exeter City: Krysiak, Baldwin, Coles, Bennett, Woodman, Sercombe, Oakley, Keohane, Davies, Gow, Parkin
Subs: O’Flynn (for Gow 73), Wheeler (for Keohane 82)
Not used: Doherty, Moore-Taylor, Dawson, Reid, Pym
Away fans: 258