Party leaders carefully script and rigorously prepare themselves ahead of each federal election debate, but sometimes, a curveball or a particularly effective delivery can significantly shift the balance of power in an election campaign.

With Conservative Leader Stephen Harper set to face Liberal Leader Justin Trudeau, NDP Leader Thomas Mulcair and Green Party Leader Elizabeth May on Thursday night, here’s a look back at some of the most memorable TV debate moments in Canadian history.

1984 - Brian Mulroney blasts John Turner over patronage appointments

Liberal John Turner inherited a big headache when he took over from the retiring Pierre Trudeau as prime minister in 1984. Shortly before Trudeau retired, he’d appointed several loyal party members to the Senate and other high-profile positions, in an act many criticized as blatantly partisan. However, Trudeau left office before the appointments received his official sign-off, and that duty was left to his successor, John Turner. Turner had been required to sign off on the appointments upon taking office, as a condition of Trudeau’s early retirement. Turner then called an election 10 days after he became prime minister.

The Liberal patronage appointments became a hot issue during the election debate, and a sore point that Progressive Conservative Leader Brian Mulroney opted to exploit.

In a pre-scripted but passionately-delivered attack, Mulroney eviscerated Turner during the debate. “You, sir, owe the Canadian people a deep apology for having indulged in that kind of practice with those kinds of appointments,” Mulroney said.

Turner defended himself by saying he had “no option” in approving the patronage appointments. However, he failed to mount a convincing argument against Mulroney’s fiery attack.

“You had an option, sir,” Mulroney said. “You could have said: ‘I am not going to do it. This is wrong for Canada, and I am not going to ask Canadians to pay the price.’ You had an option, sir, to say no, and you chose to say yes to the old attitudes and the old stories of the Liberal Party.”

Turner stuck with his meagre defence, shrugging and saying “I had no option.”

But Mulroney kept coming. “That is an avowal of failure. That is a confession of non-leadership, and this country needs leadership,” he said. “You had an option, sir. You could have done better.”

Turner’s Liberals suffered a crushing defeat at the polls that year, surrendering their hold on a majority government and falling down to 40 seats in the House of Commons.

1988 – John Turner accuses Brian Mulroney of selling out to the U.S.

The 1988 federal debate offered a chance at payback for Turner, after Mulroney shredded him in the 1984 debate. This time, Turner came out swinging at Mulroney over the Canada-U.S. Free Trade Agreement, which became the dominant issue of the campaign.

During the 1988 debate, Turner accused Mulroney’s cross-border deal of undoing 120 years of Canadian economic independence from the U.S.

“With one signature of the pen you’ve reversed that, thrown us into the north-south influence of the United States, and will reduce us, I’m sure, to a colony of the United States,” Turner said in a passionate rant at Mulroney. “Because when the economic levers go, the political independence is sure to follow,” he added.

Turner’s bluster helped resuscitate a struggling Liberal campaign that had some in the party mulling a leadership change mid-election. The Liberals still lost the election, but they more than doubled their previous seat total, rising up to 83 seats from the 40 they secured in 1984.

2000 – Stockwell Day uses a sign to call out Jean Chretien

Canadian Alliance Leader Stockwell Day attempted to put Liberal Leader Jean Chretien on the spot during the 2000 debate. Speaking to address Liberal attack ads on his party platform, Day tried to turn the tables on his opponent.

“It is a clear position: no two-tiered healthcare, and yet you, sir, have taken out an ad saying that we are not telling the truth,” Day said. While he spoke, Day unveiled a hand-written sign that read: “No 2-tier healthcare,” in a bold move that defied the rules of the debate.

But Day didn’t stop with the sign, as he pushed Chretien for a response. “I want to ask, would you do one of two things: either right now, sir, would you call me a liar please, or would you pull those ads that are wrong?” he said.

Despite Day’s showmanship, he failed to lure Chretien into answering. The Liberal Leader simply shrugged.

Chretien’s Liberals ultimately won that election by capturing 172 seats and a majority, while the Alliance won 66 seats to maintain its status as the Official Opposition.

2011 – Jack Layton questions Michael Ignatieff’s attendance

NDP Leader Jack Layton had an ace up his sleeve in the 2011 election, and he waited until the federal debate to use it.

Layton dropped a political bomb on Liberal Leader Michael Ignatieff at the 2011 election debate, questioning his attendance record in the House of Commons.

“Why do you have the worst attendance record in the House of Commons of any Member of Parliament?” Layton asked during the debate. “If you want to be Prime Minister, you’d better learn to be a Member of Parliament first.”

Ignatieff seemed taken aback by the sudden blow, as his attendance had not been a point of discussion prior to that moment. But Layton kept coming, hammering at his point by making it relatable for all Canadians. “You know most Canadians, if they don’t show up for work, they don’t get a promotion,” he said.

The attack was a significant blow against the Liberals, and against Ignatieff’s leadership. On election day, the NDP managed to snare many left-leaning voters to secure 103 seats as the Official Opposition, while the Liberals tumbled to 34 seats. Ignatieff even lost his own seat in Parliament.