Seven of the Raiders 17 were playing in their first final on Sunday against a Broncos side brimming with talent and post-season experience.
The inexperienced Raiders, led by a double from 23-year-old fullback Kaeo Weekes and a try to 21-year-old five-eighth Ethan Strange, were electric as they ran out to a 16-point lead in the second half.
But their right edge defence was tested by a red-hot Walsh after the Broncos fullback returned to the field from the sin bin and scored or set up three tries to take the game to extra time.
The Raiders thought they had done enough to secure a home preliminary final numerous times.
Walsh's two-point field goal attempt on the final siren was charged down, only for the video referee to rule Zac Hosking had made contact with the fullback's trailing leg, giving the Broncos a penalty 10 metres out.
Canberra then thought they had the result wrapped up after 10 minutes of extra time and two minutes of golden point.
Coach Ricky Stuart's rookie son Jed leaped high to bat back a Jamal Fogarty bomb, with Fogarty eventually crossing the tryline to send a sold-out GIO Stadium into raptures.
But the bunker once again intervened to deny Canberra's joy, ruling the touch went forward off Stuart's hand.
The ground went from elation to despair a minute later when veteran Ben Hunt squeezed a drop-kick off the inside of the right upright to clinch an incredible result for the Broncos.
Recovering from this finals week one loss will be a tough ask for the Raiders, despite being the league's stand-out side this season.
In the past 18 years, only two top four teams that have lost in week one have gone on to win the premiership: North Queensland in 2015 and Penrith in 2021.
"It's a pretty heartbreaking feeling," a dejected Strange told broadcaster Channel Nine after the game.
The Raiders will have less than a week to recover from the emotional turmoil and prepare for a do-or-die semi-final against Cronulla.
