A moment of madness from goalkeeper Jordan Archer set the Cobblers on their way to a defeat at Southend United.
The on-loan Spurs stopper made a bad error to give the home side the momentum in a game in which goalscoring opportunities and overall quality were in short supply.
Archer fouled Barry Corr after spilling a tame cross to give Southend a penalty, which Corr himself scored, and for which the keeper was sent off.
With 10 men, Northampton never looked like getting back into the game and did not produce a shot on target; Southend did, through David Worrall, to complete a 2-0 victory.
John-Joe O’Toole was given a starting role against his parent club and produced the first shot on target of the game, connecting nicely with a volley which Jordan Archer blocked into the path of Jack Payne, who put it wide when off balance.
Barry Corr struck a shot narrowly over the bar on the run on 16 minutes and debutant Ollie Banks fired wide at the other end.
Banks almost broke free on a long ball over the top but was held back by Adam Thompson, who was booked when a red card might have been shown.
The Chesterfield loanee skewered a decent chance into the crowd after arriving to meet a bouncing ball on his left side at the back post in what was the Cobblers’ best chance of the first half.
Kelvin Langmead had the first good one of the second half, planting a header wide from Darren Carter’s corner on 51 minutes.
Southend left-back John White made a darting run inside which ended with a fizzed shot into the side netting and Marc Richards fired a right-footed free-kick off target after Banks was hauled down on the edge of the box.
Disaster struck for the Cobblers on 67 minutes. A harmless Myles Weston cross was floated into Archer but the goalkeeper spilt it and, although Corr did not initially realise, he did collect the loose ball and took it around Archer, who put out a hand and brought the striker down.
Archer was sent off for the incident and Corr buried the penalty, high into the roof of the net.
Town faced an uphill task at that point and the game moved completely out of reach for them when Worrall tapped in Weston’s low cross 12 minutes from time.
Southend: Bentley; Thompson, Prosser, Bolger, White; Worrall, O’Toole, Timlin, Weston; Payne, Corr
Subs: Atkinson (for Weston 82), Coulthirst (for Payne 85), Deegan (for O’Toole 89)
Not used: Smith, Clifford, Deegan, Williams, Sokolik
Cobblers: Archer; Tozer, Langmead, Collins, Robertson; Murdoch, Ravenhill, Carter, Byrom; Banks; Richards
Subs: Hackett (for Murdoch 31), Mohamed (for Byrom 46), Duke (for Ravenhill 68)
Not used: Sinclair, Hornby, Moyo, Toney
Away fans: 418