Carlisle United stunned the Cobblers with a victory which ended their six-game Sixfields winning streak.
The Cumbrians ran out 2-0 winners at Sixfields with two goals in the second half, from Billy Paynter and Kyle Dempsey.
They were good value for their victory in a match in which Northampton struggled to find any fluency in attack and were picked off in defence.
As a result, they are now four points off the Sky Bet League Two play-off places with nine games left to play.
Lawson D’Ath forced Dan Hanford into the first save of the game when he got a perfect and clean connection with a diving header on the end of Chris Hackett’s swirling ball to the back post.
James Gray did well to win a free-kick on the corner of the penalty area after nutmegging Matt Young and being clattered by the right-back.
Hackett struck the dead ball into the wall and only a Tom Newey header denied the Cumbrians on the counter-attack when they had a three-on-one advantage.
A left-footed Brendan Moloney shot flew wide of the target with Hanford beaten but the right-back then made a total mess of a volley after good interplay between D’Ath and Hackett down the left.
Town made a good start to the second half and on the hour mark threw Ryan Cresswell forward as an auxiliary centre-forward.
But it was Carlisle who broke the deadlock. Jason Kennedy showed a good turn of pace to get down the left wing and pull a cross back from the touchline which was converted at the near post by Paynter.
The burly striker had only been on the field for a few minutes and the scoring touch was only his second of the game.
The visitors had a vital second goal almost immediately afterwards, with Dempsey heading powerfully home to dispatch a right-wing cross in grand style.
All three substitutions were used with attacking intent by Chris Wilder, whose team played the final 15 minutes of the game in a 3-4-3 system but still struggled to create.
Without their two best attacking players, Ricky Holmes and Marc Richards, there was always a danger that would be the case, and so it proved.
Cobblers: Duke; Moloney, Cresswell, Collins, Newey; Taylor, Carter; D’Ath, O’Toole, Hackett; Gray
Subs: Toney (for O’Toole 68), Bodin (for Collins 74), Byrom (for Carter 77)
Not used: Jalal, Perry, Diamond, Warburton
Carlisle: Hanford; Young, Grainger, O’Hanlon, Potts; Amoo, Kennedy, Corry, Dempsey; Potts, Asamoah
Subs: Paynter (for Asamoah 65), Rigg (for Corry 65), Thirlwell (for Amoo 81)
Not used: Buddle, Spiegel, Wyke, Griffith
Carlisle fans: 195
Referee: Tim Robinson