Canadian Prime Minister Justin Trudeau certainly knows how to get under Donald Trump’s skin.

Trudeau was giving a brief statement urging Trump to drop his steel and aluminum tariffs on Canadian imports at the signing of the United States-Mexico-Canada Trade Agreement (USMCA) in Argentina.

Trudeau said the tariffs remain a major obstacle to U.S. and Canadian prosperity, even under the new USMCA deal that both leaders signed with Mexico President Enrique Pena Nieto ahead of the G20 summit.

“We will not rest while those barriers remain,” Trudeau said in remarks delivered at the signing in Buenos Aires. “Canada will be as relentless in meeting this challenge as we were in updating NAFTA.”

But it was the moment was Trudeau referred to Trump as “Donald” that got everyone’s attention…including Trump himself.

Trump hates being referred to by his first name and prefers others call him Mr. Trump. As Time Magazine noted, as soon as he took office in January, 2017, the White House even updated his official biography to make sure all references read “Mr. Trump.”

“Mr. Trump followed in his father’s footsteps as a real estate developer, and he entered the world of real estate development in New York,” his biography read. “Mr. Trump has authored over fourteen bestsellers and his first book.”

Trudeau isn’t the first politician to rattle Trump by calling him by his first name.

During the presidential debates in 2016, Hillary Clinton set the tone when she greeted Trump as “Donald” and then set out to rile him up over the course of three debates, hitting all of his buttons — claiming he isn’t really as wealthy as he claims, attacking his treatment of women, and calling him a “Russian puppet.”

WATCH:

The exact moment when Canada's Prime Minister referred to President Trump as "Donald" in public – something he reportedly hates.

Then, his reaction. pic.twitter.com/ajg32OUyfu

— Muhammad Lila (@MuhammadLila) November 30, 2018

And there’s more!

There's more. There were three copies of the new NAFTA deal to sign. During the signing ceremony, President Trump appears to sign in the wrong place on one of them.

This is the exact moment everyone realized, individually with their reactions, including Prime Minister Trudeau. pic.twitter.com/Wd3CEtV73D

— Muhammad Lila (@MuhammadLila) December 3, 2018