The Cobblers succumbed to an injury-time equaliser by Wimbledon as they failed to secure the win that would have put them level on points with two of their relegation rival
Sammy Moore swept home a dramatic leveller in the fifth minute of time added on to deny Northampton a vital three points.
Darren Carter had put the home side in front with a 25-yard strike after his penalty had cancelled out George Francomb’s first-half goal and the Dons played for an hour with 10 men following Jack Midson’s sending-off.
That appeared to be enough for Chris Wilder’s men to secure the win that would have put them level on points with Portsmouth and Wycombe but still in the bottom two on goals scored.
But Wimbledon kept going until the end and got their rewards with a last-gasp equaliser.
Town had already survived a scare when Chris Hackett clumsily challenged Midson from behind in the box but referee Michael Bull waved away the strong appeals.
Wimbledon, though, were celebrating soon afterwards when a neat piece of interplay between Midson and left-winger Michael Collins saw the ball played in low from the touchline.
A covering player got a touch on it to slow the pass down but it diverted directly into the path of Francomb, who was left with the simple task of smashing home from 14 yards.
They almost doubled their lead 10 minutes later when a cross off the other wing was sidefooted against his own crossbar by Leon McSweeney from no more than two yards out.
If that was fortunate then so too was the home side’s route back to parity.
Dons keeper Ross Worner failed to punch a corner away and, after it was poked back towards the goal line, Midson appeared to clear it with a flick of the heel, but the ball had struck his hand.
Referee Bull awarded the penalty, showing Midson a red card for his part in the incident.
Carter hit the penalty better than either of his previous two this season, striking the ball with power and precision into the top left-hand corner of Worner’s net.
The Cobblers did their best to make the extra man count, spreading the ball from wing to wing and showing few signs of the panic in possession that undermined them when they had a man advantage against Mansfield.
Genuine goalscoring opportunities, though, were few and far between as the visiting defence played deep and denied Northampton any space behind.
It looked like being another frustrating game but Carter had other ideas, lashing a shot in from 25 yards with the deflection off the covering defender looping it over Worner’s head.
They had half a chance to put the contest to bed in injury time but Cristian Lopez failed to make a solid connection at the front post after Leon McSweeney had beaten his man for pace.
Although it was not a good opportunity, the failure of the team to convert it proved costly as a long ball into the box was headed into the path of Moore, who smashed home a dramatic late equaliser.
Cobblers: Duke, McSweeney, Doumbe, Diamond, Robertson, Hackett, Ravenhill, Carter, Dickenson, Connell, Sinclair
Subs: Lopez (for Connell 57), Morris (for Dickenson 57) and Horwood (for Robertson 57)
Not used: Tozer, Snedker, Deegan, Toney
Wimbledon: Worner, Fuller, Bennett, Frampton, Richards, Francomb, S Moore, Morris, Collins, Midson, Wyke
Subs: Antwi (for Frampton 74), L Moore (for Wyke 74), Arthur (for Francomb 79)
Not used: Sweeney, Pell, Brown, Nicholson
Away fans: 362