The Cobblers’ three-game winning streak came to an abrupt halt at Oxford United, who scored two dubious goals to run out victors at the Kassam Stadium.
In awful playing conditions, Clive Platt’s header looked like it would be enough to cancel out the home side’s early strike through James Constable - who looked offside as he received the ball - but the Us replied instantly with a match-winning goal through Jon-Paul Pittman.
It was a harsh outcome for Town, who had the vast majority of possession but who were ultimately beaten by a counter-attacking Oxford side and one that received the benefit of a number of debatable decisions from the match officials.
The first good chance of the game came after a corner to the home side, when Jake Robinson showed good speed to lead a counter-attack that led to a three-on-three break and finished with Robinson himself curling a shot high of the target.
Hackett then drove a left-footed effort straight at Ryan Clarke for the keeper to pat down and saw a second attempt blocked as the defenders doubled up on him.
The first goal, though, went to Oxford, and was brutally simplistic. Andy Whing cut a square pass into the six-yard box that Constable tapped in from what appeared on offside position.
After checking for the flag, Constable ran away in celebration and Oxford enjoyed a prolonged spell of possession.
The striker could - and perhaps should - have given his team a two-goal advantage soon after with a shot at a corner that hit the bar with the aid of a defensively crucial deflection.
The home side hit the woodwork a second time when Jake Forster-Caskey hit a shot from 25 yards that had Lee Nicholls well beaten, although the keeper did well to recover and keep out Sean Rigg’s follow-up effort.
Town enjoyed a lot of the ball, with the home side adopting the tactical mentality of an away team at times, and they had regular sights at goal such as the one Ben Harding got when his right-footed volley rattled into a pile of bodies on the goal line.
The second half played out in much the same way as the first, with Northampton having a lot of the ball and Oxford looking for sprightly two and three-man breaks.
Town had manufactured a number of good positions before they got their deserved goal, a very intelligent header from Platt who had done well to put Harding away on the right wing earlier in the move.
It was no less than they deserved but the rug was pulled out from under them with another goal that had more than a hint of offside about it.
The visiting defence waiting for a flag that never came and Jon-Paul Pittman raced clear before depositing a fierce shot past Nicholls.
The old cliche about a team not being more vulnerable than immediately afterwards rang true, and although the fought hard to reassert themselves in the game, it was without the same spark they had when battling back from 1-0.
In a costly final minute, Clarke Carlisle was sent off when he picked up a second yellow card for a foul that was barely a foul. It rounded off, and summed up, a frustrating afternoon.
Cobblers: Nicholls, East, Langmead, Carlisle, Tozer, Robinson, Hornby, Harding, Demontagnac, Hackett, Akinfenwa
Subs: Platt (for Demontagnac 61)
Not used: Charles, Snedker, Mukendi, Moult, Huws
Oxford: Clarke, Whing, Raynes, Wright, Capaldi, Potter, Leven, Cox, Forster-Caskey, Rigg, Constable
Subs: Pittman (for Forster-Caskey 56), Worley (for Rigg 83
Not used: Brown, Chapman, Heslop, O’Brien, Davis
Cobblers fans: 1,073