Since his surprising election last October, Trudeau has enjoyed soaring popularity as the young, hip, progressive Prime Minister of Canada. Blessed with charm, good looks, a name and a photogenic family, he has become an overnight international celebrity.
Promising "sunny ways" after a decade of the divisive politics of Stephen Harper and his Conservatives, Trudeau and his Liberals have proclaimed a new tone of civility in Parliament in their seven months in office.
But all that dissolved on the floor of the House of Commons Wednesday in a fracas both unseemly and unprecedented. At the center was a less saintly Trudeau: petulant, impatient and later, embarrassed and "unreservedly" apologetic.
The contretemps was Trudeau's worst day in office. It raises questions about his maturity, and has given his critics, who have always attacked his youth and inexperience, reason to argue that he does not have the intellect or temperament for the job.
They are wrong; he does.
But he now knows that with celebrity comes scrutiny, and he has to be more careful. He also has something to learn about the behavior -- read restraint -- demanded of the leader of a leading industrialized country.
He certainly did not look prime ministerial as lawmakers were waiting to vote on right-to-die legislation the government was trying to accelerate by limiting debate. Trudeau crossed the aisle, pushed aside one member and unintentionally jostled another, provoking an explosion of indignation.
Critics accused him immediately of "manhandling" and "molesting." Veteran politicians said they had never seen such conduct. "I was elbowed in the chest by the Prime Minister and then I had to leave," Ruth Ellen Brosseau said later. She found it "so very overwhelming" that she missed the critical vote on legally-assisted suicide.
"What kind of man elbows a woman?" cried Thomas Mulcair, the leader of the NDP.
Later, Trudeau -- who once worked as a nightclub bouncer -- was repentant. "I apologize to my colleagues, to the House as a whole and to you, Mr. Speaker, for failing to live up to a higher standard of behavior," he said Thursday. "I expect better behavior of myself ... I made a mistake. I regret it."
At the same time, the government announced that it was withdrawing the contentious motion that makes it harder for the opposition to delay government bills.
Trudeau's apology notwithstanding, the incident does indeed raise questions about the demeanor of the man who led his party from third place to power in one election, quintupling its seats. In the campaign, the Conservatives had called him "just not ready," a byword for light, shallow and callow.
"Is an apology good enough?" asked Michael Bliss, one of Canada's leading historians. "How about growing up? Does Justin Trudeau have any concept of the dignity of the office he holds?'
Up to now, Trudeau, who is 44, has had a free hand as prime minister. In his shakedown cruise, he has had the advantage of a majority government and a fragmented opposition. The two other main opposition parties are leaderless, and Canadians remain pleased that Trudeau is not the dour Stephen Harper.
So he has commanded the political stage at home and abroad like no prime minister since his father, Pierre Elliott Trudeau. He formed a cabinet of gender equity, he explained, because "it's 2015."
He romanced President Obama at a state dinner in Washington, greeted Syrian refugees in Toronto, sparred in a boxing ring in Brooklyn, hobnobbed with Leonardo DiCaprio in Davos, and discussed quantum computing at the University of Waterloo. He has taken selfies -- everywhere, with everyone. Social media is his oxygen.
But there is more to prime minister than being "the selfie king," as his critics mock him.
A moment of pique brought out a side to Justin Trudeau that Canadians had not seen. While it is too early to declare his "honeymoon" over, he has lost his halo. But in apologizing quickly and fully, he did the right thing. It shows a humility and humanity, and that will protect his brand.
At the end of a bad day, he learned some early lessons in governing, on the expectations of leadership, and the harsh discipline of power.