Saints hit the self-destruct button as they fell to a 19-13 defeat in their Heineken Cup opener at Castres.
Luther Burrell handed Romain Martial an early try and Ben Foden suffered a sin-binning as Northampton were made to pay for not turning possession into points.
Dylan Hartley’s well-taken try and Stephen Myler’s eight points with the boot did provide the consolation of a losing bonus point, but it was a missed opportunity for Jim Mallinder’s men.
Castres kept their noses in front throughout and Martial’s try allied with 14 points from metronomic scrum-half Rory Kockott gave them a valuable win.
It leaves Saints needing a big performance and a positive result against Ospreys next Sunday, but the build-up for that game will be tinged with regret.
Burrell’s intercepted pass set the tone at the quaint Stade Pierre-Antoine as Martial was presented with the ball in his own 22 and beat George North in a foot race to score.
Kockott add the extras with ease and having looked the most likely to touch down early on, Saints found themselves 7-0 down.
The away side were enjoying all the possession and, crucially, staying on the right side of referee George Clancy, who awarded them a plethora of early penalties.
And, on 30 minutes, they took the chance to get some points on the board, as Myler made the most of a tee kick in front of the posts.
But, moments later, Saints handed Castres another advantage as Foden became the eighth Northampton player to see yellow in six games this season.
The full-back came back from an offside position to obstruct a Castres attack and hung his head in dismay as he headed to the touchline.
Saints’ troubles were all of their own making and Kockott soon landed a superb penalty from halfway to put his seven points ahead at the break.
Northampton needed a response. They needed a cutting edge.
But they continued to cause themselves problems when the game restarted, giving Castres the chance to extend their lead to 10.
Ken Pisi’s poor pass gave the French side possession back and they pushed upfield to win another penalty that Kockott slotted with ease.
Saints continued to huff and puff through maul after maul, and, 10 minutes after the break, they finally blew the Castres door down.
This time it was the French side who shot themselves in the foot as Brice Mach and Kockott fumbled to gift Hartley the ball and an opportunistic try.
Myler made the conversion to bring his side to within three before Kockott increased the gap to six with a penalty in front of the posts.
Saints’ fly-half looked to reply with a long-range effort but his attempt from distance was wide of the posts.
The cavalry were immediately called for as Tom Mercey, Samu Manoa and Lee Dickson arrived to freshen up the Northampton cause.
Saints continued to turn the screw and Myler added another three points with an easy penalty, but the metronome that is Kockott replied with a one of his own.
Saints played the game in the opposition half until the end, but a late loss of possession from a lineout close to the line summed up a frustrating day as Castres claimed the win.
Castres: Palis; Martial, Cabannes, Lamerat, Evans; Tales (c), Kockott; Lazar, Mach (Rallier 70), Peikrishvili (Wihongi 51), Gray, Capo-Ortega, Bornman, Faasalele (Wannenburg 59), Claassen.
Replacements not used: Coetzee, Samson, Tomas, Grosso, Kirkpatrick.
Saints: Foden; K Pisi (Elliott 65), G Pisi, Burrell, North (Wilson 69); Myler, Fotuali’i (L Dickson 59); Corbisiero (A Waller 67), Hartley (c), Ma’afu (Mercey 59), Lawes, Day (Dowson 65), Wood, Clark, Dickinson (Manoa 59).
Replacements not used: Haywood.
Referee: George Clancy