For the second Saturday running the Cobblers conceded a stoppage-time winner as Peterborough United once again earned the local bragging rights with a latest of late goals in a 1-0 win at Sixfields.
After both sides had huffed and puffed in a hard-fought tussle, the game appeared destined to end level until Chris Forrester was left completely unmarked inside the penalty area to nod home a 92nd minute winner.
That broke Cobblers hearts and condemned them to a second successive late defeat and their second loss to rivals United in a month, dropping them down to 12th in Sky Bet League One in the process.
In front of the highest ever attendance for a league game at Sixfields - 7,675 - the Cobblers shaded a lively first-half that didn't lack in entertainment or talking points but a shortage of quality at both ends kept clear-cut chances at a premium.
Town went closest when Posh keeper Luke McGee produced an excellent double save, while the home side also had three strong shouts for a penalty turned down.
The second-half followed a similar pattern although Peterborough edged things and it was they who grabbed the late winner, sparking wild scenes among their strong away following.
It was a cagey, tight affair for the most part and a draw would probably have been fair, but if either side deserved to win, it was the visitors and now Northampton must lick their wounds and go again ahead of a testing run of fixtures.
Rob Page made two changes to the side which lost to Gillingham as Jak McCourt returned from injury to start in midfield, replacing JJ Hooper, and Alex Revell came in for Marc Richards up front.
Peterborough settled the quicker and had three early sighters on Adam Smith's goal, the first two sailing comfortably over the crossbar before Smith collected Leonardo Da Silva Lopes' low free-kick at the second attempt.
After a scrappy start, the game burst into life on the 14 minute mark when Northampton saw two strong penalty appeals turned down as both Sam Hoskins and McCourt went down in the box, before Posh broke and Tom Nichols curled a well-struck 20-yard effort a fraction wide of the top corner.
That flurry of action livened up both sets of supporters at a packed Sixfields, and the Cobblers were gradually gaining the upper-hand as Matty Taylor hit the wall with a free-kick before Zander Diamond headed too close to Luke McGee.
That was Diamond's last contribution of the game as the defender was then forced off with a head injury and replaced by former Posh man Gabriel Zakuani.
The more the first-half wore on, the more Northampton took control and dictated, and they were almost ahead just after the half-hour mark as McGee pulled off a fine double save.
The impressive Moloney darted into the box and drilled a shot that McGee parried away before the Posh goalkeeper stuck out a leg to deny Revell's follow-up effort.
The home fans grew increasingly irate with referee Darren Bond who waved away more penalty appeals for the hosts despite Posh players seemingly holding on at every corner.
Michael Bostwick headed just wide from a free-kick as Peterborough turned the tide somewhat and got back on an even keel without overly troubling Smith in the home goal.
Long-range shots provided the only real goal threat for the visitors, while Northampton ended the half on top but again clear-cut chances were at a premium, which kept the sides locked at 0-0 going into half-time.
With Cobblers struggling to get going after the restart, United started the second-half on the front foot and almost got clear on goal on two separate occasions but Nichols elected to pass instead of shoot in the first instance before the visitors spurned a great chance to counter after John-Joe O'Toole had needlessly conceded possession.
As Peterborough bossed possession, Northampton retreated back and only rarely threatened on the counter-attack with Hoskins squandering a two-on-one opportunity while Brendan Moloney fizzed a left-footer over the top from 20 yards.
Forrester's 20-yard attempt was denied by a combination of Lewin Nyatanga and Zakuani but that was one of few chances at either end.
Northampton did go close with 10 minutes remaining though when McGee tipped over Taylor's sweetly-struck free-kick to keep the game balanced on a knife-edge heading into the closing moments.
The Cobblers enjoyed the majority of possession but Peterborough carried a constant threat on the break as time ticked.
Hoskins came agonisingly close to winning it in the final minute of normal time when his deflected shot struck the crossbar, and that miss proved oh so costly in stoppage time.
With only two minutes added on, Peterborough launched one final attack and Marcus Maddison launched a deep cross into the penalty area and with Smith hesitant to come out, Forrester was left all alone to nod home a dramatic winner and send the 1,462 fans into delirium.
There was enough time to kick-time but that was about it as Peterborough held on the complete the double over their rivals.
Cobblers: Smith, Moloney, Diamond (Zakuani 25), Nyatanga, Buchanan, McCourt (Beautyman 80), O'Toole, Taylor, Hoskins, Anderson, Revell
Subs not used: Cornell, Potter, Hanley, Gorre, Richards
Peterborough: McGee, Smith, Baldwin, Tafazolli, Hughes, Lopes (Edwards 80), Bostwick, Forrester, Maddison, Coulthirst (Angol 69), Nichols (Taylor 69)
Subs not used: White, Binnon-Williams, Moncur
Referee: Darren Bond
Peterborough fans: 1,462