Jordan White's second-half header saw struggling Ross County beat Celtic 1-0 as the champions' grip on the Scottish Premiership title all but evaporated in the Highlands.
White roses highest of all to head home substitute Harry Paton's 71st-minute free-kick as Ross County, who started the match bottom of the table, recorded just their second league win over Celtic.
The visitors enjoyed 74 per cent possession, but rarely tested Ross Laidlaw in the home team's goal and as a result, John Hughes's side have moved from last place to 10th in the table.
Celtic, however, saw their recent five-match winning run come to an end at the Global Energy Stadium and sit a mammoth 18 points behind leaders Rangers, who are now seven points away from the title.
Ross County: Laidlaw (8), Naismith (7), Morris (7), Iacovitti (6), Tremarco (6), Andreu (6), Lakin (6), Kelly (7), Gardyne (9), Spittal (6), White (7)
Subs: Draper (6), Watson (6), Charles-Cook (6), Tillson (6), Paton (6)
Celtic: Bain (7), Taylor (6), Brown (6), Turnbull (7), Kenny (7), Christi (7), Rogic (6), Edouard (6), Ajer (7), McGregor (7), Welsh (6)
Subs: Griffiths (6), Elyounoussi (7), Laxalt (5)
Man of the match: Michael Gardyne
How Ross County shocked the champions
Neil Lennon's team knew nothing but a win would do against Ross County after Rangers' convincing victory earlier in the day, not that you would have known it though after a lacklustre and drab first 45 minutes.
The visitors dominated possession and territory, as expected, and yet the closest they actually came to opening the scoring was when Ryan Christie inexplicably fired over the bar after being set up by Odsonne Edouard nine minutes before the break.
However, a well-organised Ross County were not without their own moments of threat, with Tony Andreu's low drive testing Scott Bain at his near post just before the half-hour mark and the longer the match stayed goalless, the more one sensed an upset was on the cards.
Celtic boss Neil Lennon made two changes to the side that beat Aberdeen 1-0 last time out, with captain Scott Brown back in midfield, while Tom Rogic also started, with Ismaila Soro and Albian Ajeti both dropping to the bench.
Ross County made four changes from the side that lost 2-0 at Dundee United. Alex Iacovitti, Charlie Lakin, Blair Spittal and Michael Gardyne were all recalled. Defender Leo Hjelde was absent as he cannot play against his parent club, while Harry Paton, Jermaine Hylton and Billy McKay all dropped to the bench.
Hughes made a triple change with 20 minutes to go, perhaps sensing a famous win was on the cards, with the hosts immediately winning a free-kick near the left-hand corner flag. From Paton's accurate delivery to the far post, there was White to glance a header in off the inside of the far post.
Celtic threw everything they had at Ross County in the final stages, but the closest they came to levelling matters was through substitute Mohamed Elyounoussi's last-minute header which Laidlaw clawed to safety.
Man of the Match - Michael Gardyne
Gardyne was absolutely everywhere on Sunday night, with his all-action display epitomising all that was good about his side's eye-catching display in the Highlands.
The midfielder, 35, even had to take on the captaincy after skipper Callum Morris was forced off with a first-half injury, but that only seemed to inspire him even more.
And it would have been desperately cruel on the veteran had Celtic snatched an undeserved equaliser late on such had been his impressive performance.
What the managers said...
Ross County manager John Hughes: "I am absolutely chuffed to bits with the result. Credit to the boys their effort and commitment, their organisation, their willingness to put their body on the line.
"It's Glasgow Celtic we are playing against, there had to be an understanding that we couldn't be wide open.
"Tactically we were spot on and to go and score the goal, something we have been banging the drum about, all about delivery, and the delivery from Harry Paton and Jordan (White) peeling round the back, absolutely delighted.
"I thought big Jordan was different class and led the line really well."
Celtic manager Neil Lennon: "It's a strange one, it's a game we should have won out of sight.
"I think it encapsulates the season really. We have dominated the game, dominated the chances - and they weren't half-chances, they were gilt-edged - and then we have conceded another set-play, which has been a real weakness for us this season. Really, really poor. And the build-up to the free-kick has been poor as well.
"But we came here and won 5-0 and didn't play half as well as we did tonight. That's football sometimes."
Ross County travel to St Mirren, while Celtic take on Aberdeen at Parkhead, with both games kicking off at 3pm on Saturday.