Two Cheltenham goals in five second-half minutes left the Cobblers on the wrong end of a 3-2 scoreline against Cheltenham Town at Whaddon Road.
Terry Gornell scored twice and former Northampton loanee John Marquis was on target with what turned out to be the game’s winning goal.
Joel Byrom notched his first goal for the club and David Moyo scored late in the game but the home side were good value for their win, which sees them go above Chris Wilder’s side in the Sky Bet League Two table.
Wilder watched the second half at Whaddon Road from the stand and won’t have been impressed with what he saw as his side’s inability to defend set-pieces cost them dearly.
Town took their time to get going in the game but were bright once they had found their footing, Lawson D’Ath having a shot from the edge of the box blocked and the follow up well held by Trevor Carson in the Cheltenham goal.
Danny Alfei went on an overlapping run all the way to the touchline but his cross was dealt with by the home defence before Chris Hackett had two shots, the first wildly wide and the second narrowly over the bar.
Manager Chris Wilder was furious on the half-hour mark when Ivan Toney went to ground in the box as the players awaited a corner and nothing was given.
His mood did not improve when a loose Hackett pass saw Craig Braham-Barrett provide a cross which was cleared to Lee Vaughan, who thundered a volley over the bar.
And it got even worse when, in first-half stoppage time, Gornell manufactured a shot from the ground after his initial effort had been scuffed from a free-kick into the box.
Northampton made a decent start to the second half, with Alex Nicholls forcing a good low save from Carson after creating space for a shot from 20 yards.
They drew level when Byrom tucked home a clever Enda Stevens pass had cut the defence open on 49 minutes.
The goal energised them, as did the introduction of former Cheltenham winger Kaid Mohamed, who should have put the visitors in front but instead struck a post after their best passing move of the game on the hour-mark.
Gornell had a header well saved by Jordan Archer after a fortuitous break - Stevens tackled Vaughan and played the ball against him only for it to hit the corner flag and stay in play.
But they were eventually overwhelmed, with two goals in five minutes late in the game settling in despite Moyo’s late intervention.
Gornell found space to glance home a header from a free-kick at the near post before yet more poor defending of the dead ball.
Marquis won the free-kick on the right wing and then found himself with an unmissable goalscoring opportunity after Byrom’s sliced clearance had been headed back into the six-yard box.
The former Cobblers loanee needed no second invitation and then, as he had promised prior to the game, did not celebrate.
Moyo provided a platform for a late rally when he bravely headed in from in front of keeper Carson at a corner.
But, a few hopeful balls into the box aside, it did not really materialise, and the Cobblers’ run of winless league games stretched to a third.
Cheltenham: Carson; Brown, M Taylor, Elliott; Vaughan, Richards, J Taylor, Braham-Barrett; Harrison, Gornell, Bancessi
Subs: Deaman (for M Taylor 26), Marquis (for Harrison 71), Hanks (for Bancessi 81)
Not used: Haworth, Gould, Sterling-James, De Vita
Cobblers: Archer; Alfei, Cresswell, Robertson, Stevens; Hackett, Ravenhill, Byrom, D’Ath; Nicholls, Toney
Subs: Mohamed (for Hackett 46), Moyo (for Nicholls 62), O’Toole (for Ravenhill 82)
Not used: Duke, Carter, Langmead, Tozer
Away fans: 518