Colin Calderwood continues to be a lucky talisman in cup competitions for Northampton Town.
The Cobblers boss, who has been in charge at Sixfields for just two months, has seen his side win all four Cup ties in that period.
Peterborough, who became the first silver-goal-winners when the rule was brought in for the first time this season in the LDV Vans Trophy faced the other side of the coin when they exited in the same manner.
An equaliser in the fifth minute of stoppage time by Leon McKenzie, when he slotted home a cross from Andy Legg, took the tie between these local rivals into extra time, but it only earned Barry Fry's men a 15-minute reprieve as home substitute Lawrie Dudfield grabbed a dramatic winner in the 100th minute, allowing the tie to be stopped five minutes later.
The Cobblers had dominated the first half but had nothing to show for their efforts, although Richard Walker saw a 38th minute piledriver palmed away by Mark Tyler.
Posh threatened only once in the opening phase when Sagi Burton's venomous 35 yard free-kick was held by Lee Harper at the second attempt.
The Cobblers went in front in the 52nd minute. Quick thinking by Marc Richards in taking a throw in with Tyler out of his goal, enabled Martin Smith to cross, Richard Walker to flick the ball on and Joshua Low to score with a close-range volley.
This stung Posh into action and they came close to equalising on four occasions, the best effort coming in the 64th minute when Mark Arber headed Matthew Gill's corner onto the roof of the net.
Twenty minutes later the same player saw another header smack against the crossbar before Adam Newton put the rebound over.
McKenzie's goal prolonged the agony for home fans as the mists descended, but Dudfield came to their rescue with his sixth goal of the season, firing home smartly on the turn when Martin Reeves' corner was headed back across goal by Chris Willmott.