Paris - South African centre Francois Steyn and Fiji wing Nemani Nadolo grabbed two tries apiece as coach Vern Cotter masterminded Montpellier's 28-24 win over his former club Clermont on Sunday.
New Zealander Cotter coached Clermont to the 2010 Top 14 title as well as the 2013 European Cup final before taking over the Scotland job in 2014.
But there was no room for goodwill on Sunday as Montpellier went second in the table, a point behind leaders Lyon, while defending champions Clermont are fifth from bottom after a fifth defeat of the season.
Clermont had been on course for victory when they led 24-10 early in the second period before Steyn and Nadolo scored tries in the 52nd and 58th minutes to add to their scores in the first half.
"It was hard," said Cotter of his emotions. "When I got off the bus and saw all the Clermont fans, there was a bit of fun.
"But as soon as I was involved the match, I only thought about the game."
Stade Francais remain fourth from bottom after losing 20-19 to Brive who moved off the foot of the table.
Stade should have won the match when Steevy Cerqueira crossed for his team's third try two minutes from time.
However, flyhalf Jules Plisson missed the conversion and then went wide with a drop goal.
"It's so cruel because we had the chance to score at the end, but I didn't do it," admitted Plisson.
On Saturday, flamboyant Toulon owner Mourad Boudjellal likened his flying backs to the cutting edge of 21st century PlayStation technology but claimed his forwards were struggling to master 1980s' Atari games.
Boudjellal summoned up his colourful metaphor after seeing his expensively-assembled three-time European champions crash to an embarrassing 26-24 defeat at relegation-haunted Agen despite running in four tries.
"In the backs, four tries, it's a PlayStation team," said the club president Boudjellal.
"But up front, it's still a little like an Atari game. It's not yet developed, the Atari and the PS4, they don't go together. I'm not afraid to say it."
England wing Chris Ashton scored a stunning hat-trick of tries while a score from veteran All Blacks centre Ma'a Nonu in the corner appeared to put Toulon on course for a hard-earned win at a wet and chilly Stade Armandie.
But three minutes from time, Hugo Verdu, who had only been on the pitch for two minutes, knocked over Agen's fourth penalty to seal a dramatic win.
Leaders Lyon slumped to a first home defeat when title rivals La Rochelle clinched a rain-swept battle of attrition 19-15.
Pau coach Simon Mannix blasted referee Romain Poite after his team lost 23-20 at Racing 92, going down to a 78th-minute converted try when they had two players in the sin-bin.
Pau, leading 20-18, had to play the last five minutes with just 13 men after All Blacks wing Conrad Smith and fullback Charlie Malie were both yellow-carded.
Their desperate resistance was finally breached when replacement centre Marc Andreu grabbed Racing's second try.
"Romain Poite is a great referee at international level, but tonight I do not understand his decisions," said Mannix.
"If our two yellow cards were deserved, he did not give decisions in our favour. That's expensive and that pisses me off."
Tongan wing Frank Halai scored Pau's only try with All Blacks flyhalf Colin Slade booting 15 points.