Saints' clash with Bristol Bears ended in a Franklin's Gardens farce as the home side were reduced to 11 men before Ian Madigan's conversion sealed a 26-24 win for the Bears.
Piers O'Conor made the most of the depleted Northampton ranks as he cruised over, with Madigan's kick sealing a dramatic success for Pat Lam's men.
Boos rang out after Christophe Ridley's handling of the closing stages, with the referee continually refusing to award a penalty try in favour of dishing out four yellow cards.
Cobus Reinach, Jamie Gibson, Tom Wood and Alex Waller were all sin-binned before O'Conor and Madigan held their nerve to snatch the win from Saints' grasp.
It was a hammer blow for the home side, who had gone 24-19 up, registering a bonus point in the process thanks to scores from Taqele Naiyaravoro, Ahsee Tuala, George Furbank and Reinach.
But Bristol had the final say amid surreal scenes as Saints saw their seven-match Gardens winning streak in all competitions go up in smoke.
It denied Saints the chance to move to just a point behind the Gallagher Premiership top four and they must now regroup ahead of the Premiership Rugby Cup final against Saracens at the Gardens next Sunday.
Chris Boyd's men had lost David Ribbans to injury before the battle with Bristol, with Api Ratuniyarawa stepping in and James Craig on the bench.
And Bristol took advantage of the disruption as they started strongly.
Atter some patient play, the away side were able to send lock Joe Batley skipping in out wide.
The Bears missed the conversion but Saints needed an early response as the Gardens fell silent, with the Bears continuing to threaten.
And with Saints still sleeping, it wasn't long before the away side had their second as Jordan Crane got the ball down.
Luke Daniels added the extras and Bristol were 12-0 up inside the opening 18 minutes of the match.
Daniels lined up a kick from halfway soon after but his ambitious attempt didn't have the direction even though it had the distance.
Saints were struggling to awake from their slumber but they almost managed it when the rampaging Naiyaravoro eyed the line before Harry Thacker forced him to knock on.
Naiyaravoro was certainly the bright spark for Saints during the early exchanges and he was the one who finally ignited the hosts with a flying, one-handed finish in the left corner.
Piers Francis missed the conversion from out wide on a windy afternoon but Bristol had been hit by a big injury blow as full-back Charles Piutau had to be helped from the field.
Piutau's replacement, Mat Protheroe, presented Saints with a great chance before the break as he knocked on while trying to keep a Francis penalty in play.
But Saints wasted the opportunity and the score remained 12-5 in Bristol's favour at the break.
However, it wasn't long before the home side had their second try after the interval as Naiyaravoro made the initial inroads before Grayson flung the ball out to the other wing, where Tuala finished in acrobatic fashion.
Francis missed another tough touchline conversion but Saints now had the momentum and Bristol were struggling to contain them.
And Saints soon scored again as Tuala provided a moment of magic, using his power, speed and awareness to cut Bristol open before offloading to Furbank, who sidestepped in tidy fashion to finish.
Francis added the extras as the Gardens roared and Saints were now seeking the bonus-point score against a Bears team that had lost all of their early bite.
That was until they hit Saints with a real sucker punch as centre O'Conor beat Hutchinson in the middle of the field and raced forward before offloading to the onrushing Harry Randall for the score.
Daniels added the extras and Bristol led again, at 19-17, with 25 minutes to go.
But that advantage soon vanished as Reinach claimed one of his trademark intercept scores and Francis converted to make it 24-19 to Saints, who now had the try bonus point.
Bristol kicked a penalty to the corner in a bid to respond almost immediately, but Saints stood firm.
And the Bears then emulated those defensive efforts to thwart their hosts at the other end.
The game then turned into a huff and puff affair, which suited Bristol as they started to put a spanner in Saints' works.
And after Reinach was sin-binned for a high tackle with seven minutes to go, Gibson headed the same way as Saints were reduced to 13 men for the final four minutes.
It was penalty after penalty against the home side as Bristol knocked loudly on the door.
It was becoming farcical as another yellow card was shown, with Wood's departure meaning his side were down to 12 men.
And Saints were to be reduced to 11 before the end as the boos continued to ring out, with Waller sent to the sin bin.
After matching the Bears at the scrum, Saints were inevitably beaten out wide as Bristol had plenty of space against their weakened hosts and O'Conor scored.
Bristol still needed the conversion and Madigan delivered a nerveless kick from a position that was far from easy.
Referee Ridley was the most unpopular man in Northampton as the teams trudged off, but Bristol didn't care as they celebrated their big away win.
Saints: Furbank; Tuala, Hutchinson (Craig 77), Francis, Naiyaravoro (Mitchell 73); Grayson, Reinach; van Wyk (Waller 62), Marshall (Fish 56), Hill (Ford-Robinson 65); Moon, Ratuniyarawa; Wood (c), Ludlam, Eadie (Gibson 63).
Bristol Bears: C Piutau (Protheroe 32); Morahan, O’Conor, S Piutau (cc), Daniels (Madigan 68); Sheedy, Uren (Randall 49); Thomas (Woolmore 65), Thacker (Parry 77), Afoa (Thiede 72); Batley (Smith 58), Vui; Luatua, Heenan, Crane (cc) (Lam 43).
Referee: Christophe Ridley