The lead changed 13 times in a super-charged, sometimes calamitous and incredibly memorable Friday night clash at Allianz Stadium.
Christian Lio-Willie looked to have sealed the game for the Crusaders in the final two minutes of regulation time when he charged over on the short side from a scrum. The visitors were up 40-37 after Rivez Reihana nailed the conversion from the left side of the park.
But just after the restart, Crusaders winger Johnny McNicholl was ruled to have deliberately knocked on, tapping a Waratahs pass to ground just as the hosts were spreading the ball.
McNicholl was shown a yellow card, reducing the Crusaders to 14 players, and giving Harrison the chance to kick for penalty goal.
The back-up fly-half nailed the kick from just inside halfway to send an already classic match into extra time.
Once there, Harrison stepped up with his second clutch play in a matter of minutes, sending the ball over the black dot from the 22 to seal a famous win.
On the night the Waratahs honoured their champion side of 2014, the win kept the current team's finals equation solvable.
The NSW side has won only two of eight games to begin the year but can likely scrape into the top eight by claiming three of its final six.
The Crusaders, meanwhile, fell to a 1-6 record with their second loss to the Waratahs this season.
An expansive first half set the tone for the night and Sevu Reece was in everything for the Crusaders.
The right winger opening the scoring inside a minute after Dylan Pietsch had the ball raked out of his hands from the kick-off.
With his second try, Reece equalled Caleb Ralph on 52 tries, the most any man has scored for the Crusaders, and for good measure set the visitors' third up by breaking free down the right.
The Waratahs' best set play of the first half, a driving maul, helped Ned Hanigan to a try of his own and the hosts nearly scored the same way again just before the break.
Referee Nic Berry ruled there had been a hand under the ball as Charlie Gamble burrowed over, meaning the Waratahs had to be content with a 23-22 half-time lead.
The tight margin proved a sign of things to come as Harrison sealed a tight win off his boot.
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