After falling behind 10-0, Warner and the upstart Cardinals appeared pointed to lead at the break when they drove to a first down at the Pittsburgh 1 with 18 seconds remaining.
Then, a stunner.
Warner tried to throw a quick pass to Anquan Boldin, but Harrison stepped in front for the interception and took off the other way. Weaving his way along the sideline, the NFL's defensive player of the year nearly stepped out of bounds just inside the Arizona 10 and tumbled headfirst into the end zone with Cardinals receiver Larry Fitzgerald holding on for dear life.
With no time on the clock, officials decided to review the play to make sure Harrison got across the line before his knee or the ball touched down. It was a crucial decision if referee Terry McAuley had ruled him down, the half would have ended with the Steelers leading only 10-7.
McAuley already had taken away one touchdown from the Steelers after looking at a replay, the first time that's happened in Super Bowl history.
But the ref stepped out of the booth, flipped on his microphone and said, "The ruling on the field stands." Thousands of Terrible-Towel-waving fans, having already turned Raymond James Stadium into Heinz Field South, went nuts while Jeff Reed trotted out to add the extra point.
Harrison remained on his back through much of the review, but he might have just been getting a rest. The 242-pound linebacker who wasn't even drafted out of college will now be remembered for one of the greatest plays in Super Bowl history.
When the teams headed to the locker room, leaving the field to Bruce Springsteen and the E Street Band, the Cardinals looked thoroughly stunned by their change in fortune none more so than Warner, the 2000 Super Bowl MVP who revived his remarkable career in the desert.
Harrison's play eclipsed Desmond Howard's 99-yard kickoff return for Green Bay in the 1997 Super Bowl against New England. It was easily the longest interception return, beating Kelly Herndon's 76-yarder against the Steelers three years ago, when Pittsburgh beat Seattle for its first fifth title, tying Dallas and San Francisco.
Reed tacked on his second chip-shot field goal of the night from 21 yards after a third-quarter drive that consumed more than 8½ minutes. That left the Steelers just 15 minutes from that sixth Super Bowl ring one for the other thumb.
Reed's first field goal was from 18 yards, the Steelers settling for three after a TD was overturned, but that was about the only thing that went wrong for the Steelers during a dominating first quarter.
Pittsburgh had a 140-13 lead in total yards, seven first downs to one for the Cardinals and held the ball for all but 3:28 of the opening period. Pittsburgh was at the Arizona 5 when the quarter ended, and extended its lead to 10 points on the second play of the second period. Gary Russell powered over from the 1, and this time the TD stood.
But the Cardinals made a game of it with a nine-play, 83-yard drive that was highlighted by Warner's 45-yard pass to Boldin. Arizona never even got to third down, scoring on Warner's 1-yard pass to Ben Patrick, an unlikely weapon.
It was the first touchdown of the season for the tight end, who's usually confined to blocking while Warner looks to his dynamic duo of Boldin and Fitzgerald. Warner stumbled after the snap, but righted himself in time to throw a pass that was just high enough to get over the hands of Steelers linebacker Larry Foote.
Arizona looked poised for another score after Steve Breaston ripped off a 34-yard punt return to the Pittsburgh 43. But a chop block ruined that drive before it ever got started, the 15-yard penalty knocking the Cardinals back to their side of the field and eventually forcing a punt.
Through three quarters, Arizona had eight penalties for 81 yards, several of them totally unnecessary. More and more, the Cardinals looked like a Super Bowl neophyte, a doormat of a franchise through most of his history and overwhelmed by finally making it to the biggest stage.
Arizona was seeking its first NFL title since 1947 long before the Super Bowl era and was the first nine-win team to make the Super Bowl since the Los Angeles Rams 29 years ago.
Warner was 17-of-24 for 153 yards, but he had taken several big hits from the Steelers and the final play of the first half overshadowed all his gutty work.
Pittsburgh's Ben Roethlisberger was 15-of-21 for 162 yards and also had a pass picked off after the ball was deflected at the line. Bryan Robinson got his right hand on it and sent the ball fluttering into the arms of Karlos Dansby.
But that mistake was a mere afterthought after Harrison saved the Steelers at the goal line.
Roethlisberger burned the Cardinals on the opening possession, faking a handoff to the left, then rolling right and finding Hines Ward all alone for a 38-yard pass.
Pittsburgh drove to the 1 and, on third down, Roethlisberger appeared to power his way into the end zone on a scramble to the right, dragging Darnell Dockett along with him.
Cardinals coach Ken Whisenhunt, a former Steelers assistant, challenged the call, believing the burly quarterback was down before he got to the end zone. The replay appeared to show Pittsburgh center Justin Hartwig pulling Roethlisberger across the goal line, but McAulay ruled the quarterback down at the 1.
Pittsburgh's Mike Tomlin, only the third black head coach in Super Bowl history, immediately sent Reed onto the field to kick the chip shot, the shortest field goal in a Super Bowl since Roy Gerela of the Steelers punched one through from the same distance in the 1976 game one of the storied franchise's five Super Bowl titles, tied with Dallas and San Francisco for the most in NFL history.