Paris - England captain Dylan Hartley was red-carded after just six minutes on the pitch as his Northampton Saints slumped to a 37-10 defeat to Leinster in the European Champions Cup on Friday.
Hartley, who had started on the bench following autumn international duty, replaced Mikey Haywood after 50 minutes for his 50th appearance in the tournament.
However, just six minutes later, the 30-year-old hooker was heading off in disgrace for aiming a wild punch at the head of Leinster flanker Sean O'Brien.
As well as helping to condemn 2000 champions Northampton to a second defeat in three games in Pool 4 and allow Leinster cement top spot, the incident could dent Hartley's hopes of captaining the British and Irish Lions in New Zealand next year.
Northampton ended the game with 13 men when replacement forward George Pisi was yellow-carded for a high tackle.
"It's happening too much," said Saints director of rugby Jim Mallinder of his team's woeful disciplinary record.
"The players don't do it on purpose but you have to be so careful now. We have spoken about it over the last few weeks and we have had our disciplinary issues and it continues to haunt us."
Leinster's new Irish star Garry Ringrose scored the only try of the first half after just two minutes when he sprinted clear following a clever shimmy and dummy from full-back Rob Kearney.
Skipper and winger Isa Nacewa popped over the conversion for 7-0 which soon became 10-0 for the three-time champions when the 34-year-old New Zealander kicked a penalty.
Northampton's only points of the first period came via a 20th-minute penalty from the metronomic boot of Stephen Myler.
Northampton were level at 10-10 in the 45th minute thanks to a try from Samoan winger Ahsee Tuala who finished off a move inspired by former Munster centre JJ Hanrahan.
Myler added the extras before Nacewa hit a monster penalty to restore Leinster's lead at 13-10.
That signalled the entrance of Hartley but he and his fellow forwards were soon over-powered when O'Brien was barged over the try line thanks to the supporting muscle of No8 Jamie Heaslip.
Nacewa again hit the extras for 20-10 before Hartley's brief appearance came to an end for his right-handed haymaker at the head of O'Brien.
Referee Jerome Garces had no hesitation in reaching for the red card as Hartley trotted off much to the dismay of watching England coach Eddie Jones.
Leinster scored a third try when debutant Rory O'Loughlin outjumped an inattentive Ben Foden to dot down a clever crossfield kick from Ross Byrne.
Nacewa missed the extras but Leinster soon made their one-man advantage count when Jamison Gibson-Park skipped through a weary Northampton defence for a 32-10 lead and a bonus point. Nacewa added the fifth try in the last minute.
On Saturday, defending champions Saracens will seek to make it 12 straight wins in Europe when they take on English Premiership rivals Sale in Pool 3.
Saracens have not lost in Europe since they were beaten 13-9 by Clermont in the 2015 semi-final. The other game in Pool 3 pitches Scarlets against Toulon.
Other top match-ups see Leicester, in Pool 1, face Munster in a re-run of the 2002 final.
Top 14 leaders Clermont head to Belfast to face Ulster in Pool 5 while Toulouse will be seeking their first victory in Pool 2, away at Italians Zebre.