Saints were dumped out of the Checkatrade Trophy in the cruelest fashion, after cruising to a 3-0 lead early in the second period, before a resilient Northampton overturned the vast deficit.
Jake Hesketh scored a brace, while Tyreke Johnson put the visitors three goals ahead before a second-half revival from the hosts sent the game to a nervy penalty shootout.
The experience of the Cobblers shone through, edging the spot-kicks 4-2, in turn booking their place in the knockout phases of the competition and dashing the hopes of Southampton's plucky youngsters.
Saints began the game like a team intent on securing qualification from the group stages, and their bold approach deservedly earned the opening goal in the seventh minute.
Hesketh, returning from a long injury lay-off, strong-armed his defender before wriggling free and chipping a delicate finish over David Cornell in the Northampton net to give the visitors the lead.
In almost immediate reply, Cobblers' midfielder Matt Crooks lashed a shot millimetres wide, as Alex McCarthy scrambled across his goal line to cover the right side of a gaping net.
Crooks again shot marginally off target moments later, as Saints were forced to weather some momentary pressure from the hosts, who were watched on by an increasingly animated Jimmy Floyd Hasslebank.
Comfortable maintaining their slender advantage, Yan Valery was allowed to advance down the right on occasion, and fizzed a cross in the direction of target man Jonathan Afolabi on the half hour mark.
The home side escaped, with the clearance smashed downfield, as Saints once again attempted to strengthen their grip on the tie. Emboldened by a solid first period, Saints remerged with the same intensity, as Will Wood tried his luck from range.
A prelude to the second, Wood's effort sailed over, before Hesketh doubled Saints' lead in the 50th minute. The midfielder burst forward, gave Cornell the eyes, before slotting home to put the away side in ascendancy.
And the domination continued in the 58th minute, as some clever wide play from Valery was capitalised on by poacher Johnson. The fullback whipped an inviting delivery across the six yard line, where pacey Johnson arrived late and swept the ball neatly home.
A trio of changes midway through the second half appeared to spark Northampton in to life, as Sam Foley nicked a goal back in the 73rd minute, before striker Marc Richards attempted a lobbed effort which skewed off the sodden turf and dangerously wide.
Buoyed by the first life line, substitute Lewis McGugan soon reduced the deficit once again, with a well struck volley from 22 yards, before Ash Taylor headed home in stoppage time to level the scores.
David Cornell (GK)
Yaser KasimLewis McGugan (68′)
Marc Richards (C)
Matt CrooksMatt Grimes (62′)
Raheem HanleyChris Long (65′)
Alex McCarthy (GK)
Alfie Jones (C)
Jake HeskethArmani Little (74′)
Nathan TellaMichael Obafemi (60′)