South African fly-half Willie du Plessis kicked 12 points as Montpellier defeated Toulouse 27-18 in the Top 14 on Sunday, but the vanquished 19-time French champions insisted that he should not have even been on the pitch.
The 26-year-old former Golden Lions and Cheetahs back signed a short-term deal from Bayonne on Thursday night after Montpellier lost Demetri Catrakilis to an ankle injury and Francois Steyn through suspension.
However, Toulouse believe that Du Plessis was not eligible to play Sunday as he penned his contract only after a 1600 GMT deadline on Thursday had passed.
It was a dramatic day for Du Plessis who was summoned from the bench after just eight minutes to replace injured centre Vincent Martin.
"It was difficult for Willie du Plessis to come on so early in this type of match," said Montpellier coach Jake White, a fellow South African.
"But it's a good thing that he's with us."
Montpellier stayed on course for the play-offs with their win, a result which left Toulouse marooned in mid-table.
Montpellier ran in three tries to stay third although their failure to secure a bonus point was good news for leading duo Clermont and La Rochelle who are eight points ahead with eight games left in the regular season.
South African international brothers Jannie and Bismarck du Plessis were missing for Montpellier after returning home for their father Francois's funeral.
But their teammates were never in danger, building up a 13-3 lead at the interval thanks to a try from lock forward Paul Willemse on 27 minutes and eight points from the boot of Willie du Plessis.
Fijian winger Timoci Nagusa extended Montpellier's lead on 43 minutes with an interception try with centre Yvan Reilhac adding a third score 20 minutes later.
Toulouse scored second-half tries from Census Johnston and Alexis Palisson but their failure to boot three penalties proved costly.
Castres are fourth after a deserved 29-17 win at Bordeaux-Begles.
The visitors had only scored seven tries on the road all season but that haul reached double figures thanks to first-half tries from Horacio Agulla and Anthony Jelonch with Marc-Antoine Rallier crossing after the break.
Julien Dumora, a last minute stand-in for Argentine fly-half Benjamin Urdapilleta, added 12 points with his boot.
Castres manager Christophe Urios commented: "The two important things here were the team spirit, you saw a team that knuckles down in the face of a storm, we never let up, and then the physical aspect, we broke them on that front."
For Bordeaux the defeat stretched their winless run to seven, leaving them ninth, seven points shy of the play-off positions.
Despite their two first-half tries from Luke Jones and Yann Lesgourgues, Bordeaux lacked bite as Castres controlled the game to boost their play-off chances.
On Saturday, defending champions Racing 92 put Dan Carter's drink-driving shock behind them with a 33-25 win over Brive as Clermont grabbed pole position after a 46-27 victory over rock-bottom Bayonne.
Clermont's six-try win moved them above La Rochelle who earlier downed Stade Francais 37-18.