A much-changed Celtic suffered the first defeat of their Europa League campaign as second-half goals secured a 2-0 win for CFR Cluj in northwestern Romania.
Andrei Burca rose to head the hosts in front three minutes after the restart before Damjan Djokovic stabbed home a second with 20 minutes remaining as Cluj qualified for the knockout stages of a European competition for the first time in their history.
Dan Petrescu's side secured second place in Group E while Celtic were denied a maiden unbeaten European group-stage campaign.
The visitors finish the group on top with 13 points, one ahead of second-placed Cluj and attention will now turn to the draw for the last 32 on Monday in Nyon, with Celtic among the seeded teams.
CFR Cluj: Arlauskis (7), Susic (6), Burca (7), Cestor (7), Camora (7), Djokovic (8), Bordeianu (7), Culio (7), Deac (7), Traore (7), Omrani (6).
Subs: Luis Aurelio (6), Golofca (6), Hoban (n/a)
Celtic: Gordon (7), Bauer (6), Jullien (5), Bitton (6), Bolingoli (6), Robertson (7), Ntcham (6), Sinclair (6), Morgan (6), Johnston (5), Griffiths (6).
Subs: Bayo (5), Ajer (6), Dembele (n/a).
Man of the match: Ciprian Deac.
How Celtic's winning run came to an end
Lennon said following the defeat: "Overall, we've had an amazing campaign and I'm happy to have used the squad tonight because we have so many important domestic games coming up."
The Celtic boss was also happy with the performance of 18-year-old Scott Robertson on his senior debut, while at 16 years and 293 days old, Karamoko Dembele became the youngest player to ever play for Celtic in major European competition.
The Scottish champions were already assured of their place in the last 32, and this was remarkably the fourth time these two sides have met this season, first meeting at the Dr. Constantin Radulescu Stadium on August 7 in the Champions League qualifiers - but Giedrius Arlauskis, the Cluj goalkeeper, was rarely troubled.
Despite having already sealed top spot, 1,000 Celtic supporters made the trip to Transylvania hoping to witness the club's first win in Romania, but it was the hosts - with the extra incentive of needing a point to secure second spot - who created the best openings of a cagey first half.
Lacina Traore, the giant Ivory Coast striker once on loan at Everton, led the line well and set up Billel Omrani with a cute reverse pass after 18 minutes but Craig Gordon raced from his line to smother the shot.
Lennon made nine changes, handing a debut to 18-year-old midfielder Scott Robertson while Grant Savoury joined Karamoko Dembele on the bench.
Boli Bolingoli returned having not featured since the 5-2 win over Hibernian in early November, Leigh Griffiths started up front while Olivier Ntcham captained the side.
Celtic's best efforts came from distance, as after Nir Bitton's speculative drive was comfortably kept out by Arlauskis, Mikey Johnston came within a yard of finding the top corner when his dipping shot from outside of the box deflected just wide.
But Cluj emerged from the second period with greater urgency and duly broke the deadlock when Burca lost his marker Boli Bolingoli to head home Ciprian Deac's corner.
Gordon appeared to still be reflecting on that incident when a sloppy pass out from the back presented the ball straight to Deac, but the stand-in Celtic goalkeeper made amends with a fine one-handed save around the post to ensure Cluj didn't celebrate a quickfire second.
Celtic showed resilience earlier in the group campaign, coming back from a losing position away to Rennes, and both home and away to Lazio, but after Leigh Griffiths and Mikey Johnston spurned half chances either side of the hour, Cluj pulled away with 20 minutes remaining.
Emmanuel Culio's inviting low cross from the left was diverted back across goal by Deac and Djokovic was on hand to guide the ball beyond Gordon as Celtic's run of 12 consecutive victories in all competitions came to an end.
What the manager said...
Celtic boss Neil Lennon: "We're disappointed to lose the game but in the main very pleased with what I got out of it. Some of my players needed the games and needed the 90 minutes.
"The likes of Griffiths, Bitton, Bolingoli, Bauer and Bayo getting time on the pitch has given them minutes in their legs now. These are huge games in the context of our season because the championship is our priority.
"I thought Ntcham was unbelievable. To give him the captaincy and then for him to take on the responsibility the way he did was excellent. I thought his all-round game tonight was absolutely top class."
Celtic lack bite in Transylvania
Analysis from Sky Sports' Ben Grounds:
Revenge was a motivating factor in the reverse fixture as Celtic powered to a convincing 2-0 victory - but in Romania on Thursday night, the onus was on Lennon's squad members to take their opportunity to impress.
The Celtic manager had called on his players to express themselves with the rare luxury of not having anything to play for on matchday six, but his shadow team, including Scott Sinclair and Nir Bitton, didn't seize their chance.
Lennon was sensible in removing Christopher Jullien, the French centre-back, at half-time having conceded five fouls. In avoiding a yellow card, he is now available for the first leg of the last-32 tie in the new year.
But Cluj's proud home record, having won 10 home games in a row since losing 1-0 to Slavia Prague in Champions League qualifying in August, was rarely troubled. Heading into the hectic Christmas period, Lennon won't have been given too much of a selection headache.
Man of the match: Ciprian Deac
Lennon opted to leave first-teamers Tom Rogic, Fraser Forster, Callum McGregor, Ryan Christie and Odsonne Edouard all at home, and Deac was allowed to showcase his undoubted ability against Celtic's second string.
Deac provided both assists, meaning the 33-year-old Romanian has been directly involved in five of CFR Cluj's Europa League goals this season (two goals, three assists), providing the assist for each of the last three.
- Including qualifiers, Celtic have failed to score for the first time in Europe this season in what is their 14th game. They last drew blanks in European competition in both last 32 Europa League matches against Valencia last season.
- CFR Cluj have won three successive home games in European competition for the first time after winning each of their Europa League group stage home matches.
- At 16 years and 293 days old, Karamoko Dembele is the youngest player to ever play for Celtic in major European competition (excluding qualifiers).
The draw for the last 32 of the Europa League takes place on Monday, December 16 in Nyon, with proceedings set to start at 12pm, UK time.
Celtic return to Scottish Premiership action on Sunday when they host Hibernian at Parkhead; kick-off is at 3pm.