The Cobblers lost for the 11th time away from home, at Bradford City, but held on to their third-place position because of results elsewhere.
Town were sunk by Nahki Wells’ sixth goal against them, a simple tap-in after a mix-up between debutant defender Nathan Cameron and goalkeeper Lee Nicholls.
It was enough to secure a Bradford win in a game dominated by the whistle of referee Trevor Kettle, who had a truly awful game in the middle.
But defeats for Cheltenham, Rotherham and Exeter City mean Northampton remain third in league two with just four fixtures remaining, two of which are at home.
Clarke Carlisle found space at the Cobblers’ very first set-piece of the game, glancing a header narrowly over the target from a launched Ben Tozer throw.
A period of raised tempers then saw the visitors pick up two bookings down their left-hand side, Ishmel Demontagnac having his name for dissent and Joe Widdowson joining him - somewhat harshly - for a collision with Stephen Darby.
The first 25 minutes ended with a goal from an inevitable source but by a calamitous method.
A long hook over the top bounced behind the defence, Lee Nicholls came out of his area to meet it and Nathan Cameron headed the ball away from him, leaving Wells with the simple task of tapping into an unguarded net.
Wells provided James Hanson with a square pass to make it two after outsprinting Carlsle down the inside-left channel but it was a fraction behind the big striker and he swiped at thin air.
Adebayo Akinfenwa came on for the troubled Cameron at half-time but the game continued in the same manner, Nicholls blocking a fierce shot from Kyel Reid moments after the restart.
Ben Harding had the best chance of the game for Northampton when he fired a volley across goal in the 73rd minute but keeper Jon McLaughlin somehow managed to keep it out with an excellent, acrobatic save.
Hackett skimmed a shot that drifted wide with the aid of a deflection but just as the away side attempted to build momentum their flow was stemmed by the referee’s whistle.
He booked Akinfenwa seemingly on a whim, and then penalised the striker when McLaughlin came right through him to punch a cross away.
When Kettle was not hindering Northampton they were usually their own worst enemy.
Roy O’Donovan lashed wide after being played into space by Robinson and a John Johnson header at a free-kick that he had initially own on the right wing was powder puff at best.
Bradford: McLaughlin, Darby, McArdle, Davies, Thompson, Ravenhill, Jones, Reid, Wells, Hanson
Subs: Hines (for Thompson 72), Atkinson (for Wells 90+1)
Not used: Duke, McHugh, Connell, Nelson, Doyle
Northampton: Nicholls, Johnson, Cameron, Carlisle, Collins, Widdowson, Hackett, Harding, Tozer, Demontagnac, O’Donovan
Subs: Robinson (for Demontagnac 34), Akinfenwa (for Cameron 46)
Not used: Snedker, Guttridge, Platt, Hornby, Dias
Away fans: 413