The Cobblers were beaten by a superb late free-kick by Alex MacDonald as they slipped to a 2-1 home defeat to Burton Albion.
Midfielder MacDonald rattled home from fully 30 yards to settle a contest which Ryan Cresswell had levelled with his second goal in a week minutes earlier.
Former Town loanee Jacob Blyth had earlier put Burton in front but Northampton will feel aggrieved they did not get something out of the game, having had several good goalscoring chances, especially in the first half.
There was an emotional return to the Sixfields turf for Alex Nicholls, who was named in a starting line-up for the first time in almost two years after recovering from a badly broken leg suffered against Port Vale in October 2012.
The striker came in as one of two changes from the league game at Wycombe, with Aston Villa loanee Enda Stevens given his debut at left-back in place of Gregor Robertson.
Town made a slow start to the game and did not have their first sight of goal until the 15-minute mark when Kaid Mohamed hit a low shot that was well saved by Jon McLaughlin.
Ivan Toney and Chris Hackett had efforts blocked and saved respectively after a fine Nicholls cross had unsettled the visiting defence.
Phil Edwards made a simply superb defensive header under pressure from Toney to prevent what would have been an almost certain goal from Ben Tozer’s neat cross.
Mohamed then blazed over with the goal at his mercy after another Tozer cross in had been missed by goalkeeper Jon McLaughlin.
Northampton had been the better team in the first 45 minutes but went in at the break behind, with former striker Blyth making no mistake from 12 yards after a clipped pass had evaded the centre-backs.
Nicholls found himself in a half-decent scoring position soon after the restart when a long ball was headed into his path by Toney, but his finish - from 25 yards - was lacking and flew well wide of the mark.
Burton should have doubled their advantage when Robbie Weir fired in a low volley which Archer did well to push away to the relative safety of a corner and Adam McGurk headed over when well placed shortly afterwards.
With Burton, as was to be expected, taking their time over restarts, it looked like it might be a bad day at the office for the Cobblers.
But the visitors failed to deal with a corner, the ball was smuggled from Lee Collins to Cresswell and the centre-back smashed an unstoppable shot past McLaughlin to draw Northampton level.
It looked like it would be enough for a point but MacDonald had other ideas with just four minutes left to play.
Burton got the benefit of the doubt for a dubious free-kick which was despatched into the top corner by midfielder MacDonald with great ferocity.
Cobblers: Archer; Tozer, Cresswell, Collins, Stevens; Hackett, Byrom, Ravenhill, Mohamed; Nicholls, Toney
Subs: D’Ath (for Mohamed 50), Alfei (for Tozer 67), Langmead (for Nicholls 76)
Not used: Carter, Snedker, Robertson, Moyo
Burton Albion: McLaughlin; Edwards, Mousinho, Sharps, Taft; Weir, Palmer, MacDonald; Akins, Blyth, McGurk
Subs: McCrory (for Taft 67), Harness (for McGurk 76), Slade (for Weir 84)
Not used: Bell, Lyness, Knowles, Austin
Away fans: 339
Referee: Gary Sutton