Their precious sequence of league clean sheets fell in the north-east, and so too their hopes of going the entire SPFL Premiership season unbeaten. Neither was surrendered lightly.
It took a tremendous effort to get the ball in the net against them for the first time in the league since November, and when they were eventually beaten it was by the one team in Scotland who have been a thorn in their side more than once this season. Celtic also went down after playing with ten men for 78 minutes because of Virgil van Dijk's early red card.
This was a vibrant, joyous result for Aberdeen even if their performance lacked the certainty and verve they had shown when winning at Parkhead 17 days earlier in the William Hill Scottish Cup.
They powered their way into a commanding 2-0 lead at half-time and then did not seem sure about what to do next. Celtic came back at them in a vigorous, proud effort to defend their unbeaten run but it was beyond them.
Celtic will now complete the formality of winning the league without a "peg" to sell their games to fans. With no other team near them - even after this, Aberdeen are 21 points behind - their toughest opponent will be boredom.
They call Fraser Forster the great wall and at last his extraordinary sequence without conceding in the SPFL Premership hit a wall of its own. The great run ended with an appropriately great goal. After 1255 flawless league minutes, Forster was beaten by the sort of shot he could have faced a hundred times without stopping. Jonny Hayes lashed in a glorious strike to end the run of clean sheets and, when Adam Rooney doubled the lead, it immediately looked like the unbeaten league run would crumble too.
Celtic were down to ten men when the game was in its infancy. Van Dijk's red card was solely down to Peter Pawlett's pace. Efe Ambrose had given the ball away and when Aberdeen swarmed on to them, Rooney fed the ball for Pawlett. He then knocked it ahead of himself to burst towards the box and his pace seemed to surprise van Dijk, who brought him down just outside the area. Celtic reorganised well, Charlie Mulgrew withdrawing from left midfield to centre-half as the shape altered to 4-2-1-2. For a while they still seemed composed and assured, and Nir Biton forced a fine save from Jamie Langfield, but gradually Aberdeen emerged from their slow start and started to ask questions of the depleted Celtic team. When Hayes flashed over a terrific ball into the goalmouth, Rooney seemed likely to score but could not connect with a stretching effort.
Hayes' goal was simply fabulous. Niall McGinn squared the ball to him and he set his sights and unloaded an unstoppable, rocketing shot past Forster into his top corner. And so the Celtic goalkeeper began his next run of minutes without conceding: this time he managed four of them. McGinn whipped a cross into the heart of the Celtic penalty area and with no Ambrose-van Dijk pairing to mop up, Rooney got himself into a pocket of space between Ambrose and Mulgrew and glanced a header inside the far post.
Neil Lennon sent on James Forrest for Kris Commons at half-time, injecting pace. Within 15 minutes he had scored, and Celtic deserved that. They dominated the start of the second half just as they had the beginning of the first, and they should have had a penalty when Mark Reynolds handled the ball from Adam Matthews' long throw into the box. Only a minute after that Celtic pressed again and Scott Brown laid the ball off to Forrest to place a shot high to the top corner.
When the first half began, Aberdeen seemed to hold all the advantages: a 2-0 lead against ten men. But Celtic competed like lions to preserve their unbeaten run. Aberdeen were tame. Their terrific pace and menace on the counter attack meant Celtic had to be permanently aware of being suddenly exposed, but Aberdeen looked more inclined to sit on their lead than force themselves to seek more. Excitement in the home stands was gradually replaced by frustration and anxiety as Forrest's goal suddenly looked like it might be the platform for Celtic to steal the result Pittodrie was desperate to savour.
Only much, much later, when the game entered stoppage time, did they find their voice again. At last McGinn and Pawlett broke clear and promised a third but the former's cross was overhit. There were a few around the ground who had been licking their lips at half-time, anticipating that three or four might be put past the vulnerable champions.
Celtic played far too well for that to be realistic. For Aberdeen to beat Celtic, and feel that they might have done so more convincingly, showed exactly how far they have come.
Not since 1990-91 had they beaten Celtic home and away in the same season, let alone the same month.