Celtic's Europa Conference League campaign hangs by a thread after being dealt a serious body blow by Bodo/Glimt.
The Norwegian champions dismissed concerns over their readiness to compete as they ran out deserved 3-1 winners at Parkhead.
Kjetil Knutsen's side are still in pre-season with their domestic campaign not kicking off until April but they turned in a streetwise performance laced with quality to take control of the this last 16 play-off.
Runar Espejord opened the scoring just six minutes into his debut for the visitors before Amahl Pellegrino doubled their advantage after the interval.
Daizen Maeda reduced the arrears as time ticked away, only for Hugo Vetlesen to smash home a decisive third.
Ange Postecoglou 's side have been in sparkling form but this result brings to an end an unbeaten run which stretches back to November.
Since then, Celtic have regularly been inflicting early pain on their opponents with blistering starts but they were handed a taste of their own medicine in the opening minutes.
Bodo/Glimt, casting aside those suggestions they would be undercooked, matched the visitors' early pace and were rewarded with a quite beautiful team goal as Espejord stabbed beyond Joe Hart following a sweeping move that started almost on their own goal-line.
It wasn't the opening Celtic Park expected, with the visitors settling into a nice early rhythm before the hosts could warm to the task.
Celtic did begin to edge possession and territory as the half progressed, but without testing keeper Nikita Khaikin, despite some neat build-up through midfield.
Ultimately, however, it lacked a killer touch and culminated in a comfortable opening period for Knutsen's team.
Celtic re-emerged with purpose and for all their neat build-up, it was a route one approach that so nearly yielded an equaliser.
McGregor lofted a ball over the top from deep but Maeda couldn't find a way around Khaikin, who stood tall one-on-one with the striker.
It quickly proved a pivotal moment as Bodo/Glimt suddenly turned threatening once more.
Ola Solbakken, not for the first time, left Greg Taylor for dead on the right and his cross was cleverly flicked on by Espejord for Pellegrino to control and finish.
Postecoglou rung the changes, sending on James Forrest, Giorgos Giakoumakis and Reo Hatate as Celtic stuck manfully to their task.
They were eventually rewarded when Maeda neatly flicked home a near post header to bring Celtic Park back to life.
It was to be but a momentary reprieve.
Bodo, unshaken, raced back up the other and Vetlesen sent a deflected effort hurtling past Hart from around 20 yards.
Here's five talking points from Parkhead:
Bodo/Glimt flip the script
The expectation, which was fuelled in their own camp, it must be said, was that the visitors would arrive in Glasgow well below peak sharpness.
Celtic have very much written the script early in domestic matches of late and there was a feeling they could perhaps blitz Bodo/Glimt in the early stages.
But it was the home side who were caught cold as Celtic Park quickly discovered this would be no walk in the park.
Knutsen's team carved out a delightful opening goal and looked comfortable and content to try and play through Celtic's press.
It set the tone for an impressive, disciplined display.
Midfield aggression key
Celtic have grown accustomed to having things mostly their own way in the Premiership but they were met here by a Bodo/Glimt midfield in aggressive mood.
Knutsen deployed three real athletes in Ulrik Saltnes, Elias Hagen and goalscorer Vetlesen, who set about disrupting Tom Rogic and Matt O'Riley in the advanced midfield positions.
But the visiting trio could also play and it made for intriguing battle in the engine room throughout.
On the other hand, the game largely passed Tom Rogic by and it looked for all the world that Hatate would enter at half-time.
By the time he did, Celtic were two goals down.
Wingers subdued
Much Celtic's attacking play focuses on isolating wingers against full-backs, with Jota and Liel Abada running riot in recent weeks.
It's a tactic their visitors for the evening were clearly prepared for given how ineffective their wingers proved to be.
Bodo/Glimt's left-back, Brice Wembangomo, was particularly eye-catching on the evening.
The 25-year-old was composed, effective on the ball and, crucially, diligent in defence as he kept the in-form Abada largely quiet.
Help from his wide attackers also ensured the Norwegians were not exposed on the flanks in the brutal manner Rangers were just a few weeks ago.
Mountain to climb
Make no mistake, this was a result and performance few inside Celtic Park will have anticipated.
Postecoglou's side have been flying since the New Year but here they came up against a very capable outfit who, it must be said, did a bit of a number on them.
Pre-match talk contained notions of Celtic making a real in these knockout stages but now they're simply fighting for survival next week in Norway.
The removal of the away goals means this defeat isn't quite as catastrophic as it once would have been, but they really do still have a mountain to climb.
Reality check
In hindsight, perhaps it was no surprise that a side who gave Roma a 6-1 chasing not so long ago might have a bit about them, even if they are still in pre-season mode - allegedly.
Celtic have been utterly sparkling for long periods of games in recent weeks but tonight was a reminder they are still a work in progress.
It's hard to label this result as too much of a backwards step, more a reality check that Postecoglou's promising team still have a way to go to become continental contenders.
Bodo/Glimt were tactically disciplined, used the ball well and when they did make mistakes, Celtic couldn't punish them.
Can they respond a step up a level in the return leg? They simply have to, or it's curtains on Europe for another season.
