A dramatic last-gasp winner from Darren Dods gave Dundee United its first victory over Celtic in 10 years.
In the 71st minute of an enthralling Scottish Premier League encounter, former Tangerines' midfielder Barry Robson sent Nicky Weaver the wrong way from the penalty spot after United skipper Dods had clumsily tripped him inside the box.
It looked like Celtic would go two points clear of Old Firm rivals Rangers but the Tangerines had other ideas.
With seven minutes remaining United substitute Jon Daly, playing his first competitive game since April following a knee injury, headed the equaliser from Danny Swanson's corner.
And with seconds of normal time remaining, Swanson threw another corner into the box to allow Dods to make up for his earlier mistake and crash the ball into the net for a 2-1 upset.
With Hoops midfielder Aiden McGeady shrugging off his midweek World Cup play-off disappointment with the Republic of Ireland, Celtic looked that bit sharper in the early stages.
In the eighth minute an Andreas Hinkel drive from 25 yards drew a fine save from Weaver before Dods eventually cleared the danger.
Minutes later, McGeady sent Marc-Antoine Fortune racing clear down the right but the Parkhead striker was too hasty in sending his cross into the United penalty area and found no takers.
The Taysiders looked promising when they broke but their threat continually evaporated at the edge of the Celtic penalty area.
It was Celtic which was getting its shots away and in the 16th minute, after Dods had been penalised for a challenge on Scott McDonald 30 yards from goal, Robson fired the free-kick over the bar.
McGeady set up Fortune with a delightful cutback following a strong run down the left but the former West Brom striker hesitated and his shot was blocked for a corner.
More McGeady magic in the 37th minute, this time wide on the right, saw him ease past Dods but his cross was mishit by McDonald 12 yards out and the ball sped to safety.
Then, as Celtic pinned United back in its search for the opener, Hinkel all too easily burst into the United box from the right but his cross-cum-shot was blocked by Weaver on his line.
United came close for the first time three minutes after the break.
Wide man Jennison Myrie-Williams slalomed his way through the Celtic midfield but from 25 yards he dragged his left-footed shot wide of the target.
Seconds later at the other end, Weaver blocked a close-range shot by Fortune, who had been set up by Gary Caldwell's chip across goal.
Celtic goalkeeper Lukasz Zaluska made a spectacular save from United midfielder Prince Buaben and from the resulting corner, striker Damian Casalinuovo headed off the bar after being set up by Dods.
Georgios Samaras replaced Fortune in the 58th minute as Celtic manager Tony Mowbray looked for some inspiration from the Greece international, who had helped his side qualify for the World Cup earlier in the week with a win in Ukraine.
United responded and David Goodwillie and Swanson came on for Danny Cadamarteri and Craig Conway three minutes later.
Moments later Casalinuovo was set up by Goodwillie but failed to connect just eight yards out.
The tension increased by the minute as both sides went all out for a winner and Celtic was handed the opportunity from the spot in the 71st minute when Dods clumsily tripped Robson in