Paris - Former European Cup champions Northampton's chances of reaching the knockout stages of this season's competition were left hanging by a thread on Saturday as they fell to a surprise 41-22 defeat by French side Castres.
The Saints - beaten finalists in last season's competition and who last went out at this stage eight years ago - didn't do themselves any favours in being reduced to 14 men twice in the match as Castres wing Romain Martial scored two tries - his first for the club since he joined from Narbonne two years ago.
Romain Teulet scored 21 of their other points with his boot to help his club to their first victory in any rugby since October 22.
There was something of a surprise in another of the early matches as Italian side Treviso beat ailing two-time European Cup finalists Biarritz 30-26, set up mainly on an explosive first-half which saw them score three tries and lead 24-12 at the break.
Castres took an early lead against their English opponents with a try by Steve Malonga but Saints were back on level terms soon afterwards as flanker Mark Sorenson burrowed over and Steve Myler added the two points.
The hosts spoilt a great chance to score another try a minute later as they made inroads into the Northampton 22 but an infringement cost them a penalty which allowed the English side to clear - but when they too conceded a penalty and Teulet slotted it over to give Castres a 10-7 lead after 20 minutes.
Teulet - a prolific points scorer over the past few seasons and nicknamed 'RoboCop' for his singular style of preparing for penalty kicks - added another penalty to extend the lead to 13-7.
However, in an open and entertaining if error-strewn encounter it was the visitors who grabbed the second try as winger Jamie Elliott went over in the 32nd minute after being brilliantly set up by Samoan George Pisi.
Myler was unable to convert from the touchline to leave them a point adrift of Castres, who went four points ahead as Teulet landed another penalty.
Myler cost his side even more dearly just before half-time when he was rather harshly yellow-carded for a high tackle.
Almost immediately Castres added another try as wing Martial ran on to former All Black Chris Masoe's brilliant grubber kick to touch down for his first try for the club - Teulet converted to make it 23-12 at half-time.
However, the Saints rebounded after the break before Myler rejoined the fray as Elliott scored his second try of the game intercepting Ibrahim Diarra's pass deep inside the English side's 22 and ran it back the length of the pitch - Tom May failed to convert.
However, just as Myler did return Teulet added another penalty and give the hosts a nine point cushion with half-an-hour remaining.
The Saints were right back in it though as after good work by Pisi and Ben Foden American No 8 Samu Nanoa went over to at least guarantee a bonus point for tries scored - Myler failed to convert to leave them trailing 26-22.
Saints hopes, however, suffered a hammer blow in the 63rd minute as Tom Wood was sin-binned - a week after being red carded - and Teulet added the three points to leave the English outfit requiring a converted try to draw level.
That task became even more demanding as Martial went over for his second of the afternoon, the remarkable Teulet added the conversion from the touchline - Martial for good measure set up Diarra for another try to round off a memorable afternoon.
European Cup results on Saturday:
Castres 41 Northampton 22
Scarlets 14 Munster 17
Racing Metro 14 London Irish 34
Treviso 30 Biarritz 26
Connacht 10 Gloucester 14
Playing later (kick-off GMT)
Saracens v Ospreys (18:00)
Played on Friday:
Cardiff Blues 25 Edinburgh 8
Ulster 31 Aironi 10
Harlequins 10 Toulouse 21
Playing on Sunday (kick-offs in GMT)
Bath v Leinster (12:45)
Glasgow v Montpellier (12:45)
Clermont v Leicester (15:00)