Albian Ajeti produced a strong contender for miss of the season as Celtic slipped up again in the Scottish Premiership with a 1-1 home draw against Dundee United.
The hosts made the perfect start as Liel Abada rose to head home his third Scottish Premiership goal since joining the club (16).
But their lead lasted just two minutes as Ian Harkes rose to meet Ilmari Niskanen's cross to beat Joe Hart.
David Turnbull, Tom Rogic and Jota all hit the bar for Celtic but they failed to reproduce the form that had seen them score 26 goals in seven consecutive home wins, and have now dropped more points than they have gained in the league.
Celtic were forced to settle for a point which keeps them sixth in the table, six points off leaders Rangers. Dundee United remain one point and one place above them.
Celtic: Hart (6), Juranovic (5), Carter-Vickers (7), Starfelt (6), Ralston (6), Rogic (7), McCarthy (6), Turnbull (6), Abada (7), Ajeti (6), Jota (8).
Subs: Soro (6), Montgomery (6).
Dundee Utd: Siegrist (7), Kerr Smith (7), Edwards (7), Freeman (7), McMann (7), Niskanen (7), Harkes (7), Levitt (7), Fuchs (6), Pawlett (8), Clark (7).
Subs: Sporle (6), Appere (n/a).
Man of the match: Jota.
How Celtic stuttered again
Ange Postecoglou spoke of his admiration for Tam Courts, who arrived at Celtic Park having made a promising start to his Dundee United reign after stepping up from a role in the academy in the summer.
In spite of their recent struggles, Celtic began full of confidence and ought to have been ahead when Ajeti met Anthony Ralston's cross but contrived to direct his headed effort over the bar.
The hosts were without five attacking players - Giorgos Giakoumakis, Mikey Johnston, Karamoko Dembele, Kyogo Furuhashi and James Forrest - but they found the breakthrough inside just 16 minutes.
Abada had already been denied by a last-ditch tackle by Ryan Edwards but he made no mistake when the lively Jota found space to cross from the left, and the Israeli forward glanced his header beyond Benjamin Siegrist.
You felt at that stage, with their nerves settled, Postecoglou would register a convincing win but their lead was short-lived.
- The only two Celtic changes from their defeat at lowly Livingston were in the backline, where Boli Bolingoli and Stephen Welsh made way for Anthony Ralston and Carl Starfelt.
- Nicky Clark was given the task of replacing the injured Marc McNulty, who limped out of the midweek League Cup defeat by Hibs, with two other changes made by Tam Courts. Dylan Levitt came in for Calum Butcher in midfield, while the experienced Charlie Mulgrew was replaced by Kieran Freeman.
Niskanen escaped down the left flank and with a fine cross of his own picked out Harkes' drifting into the box at the near post to power his header past Hart for an instant reply.
Celtic had only lost one of their previous 18 league meetings with Dundee United, winning seven of their last eight against them, and they very nearly restored their lead within two minutes.
Jota combined well with Abada again, with the latter reversing the ball to Albian Ajeti only for the Swiss striker to lift his shot over the bar from four yards with Siegrist stranded.
Dundee United had lost 10 of their previous 11 away league visits to Celtic, drawing the other 1-1 in November 2013, but they had strong claims for a penalty midway through the opening period.
Referee Kevin Clancy was unmoved as after Carl Starfelt was brushed off the ball by Nicky Clark, the Dundee United striker appeared to be bundled over in the area by Cameron Carter-Vickers.
Adam Montgomery replaced Juranovic and initially struggled with his passing, although he was far from the only Celtic player to do so.
Jota continued to be a threat, though, and saw efforts turned past by Siegrist and Ryan Edwards.
Turnbull almost lifted an audibly frustrated home support in the final minute of the half when he turned and unleashed a powerful left-footed strike from 25 yards that hit the underside of the bar. Ajeti tucked home the rebound but he had pushed Edwards to get to the ball and the goal was rightly disallowed.
Ismaila Soro came on for the second half for James McCarthy, who had picked up a knock following a late tackle from Jeando Fuchs, but also generally struggled to inject the tempo required by Postecoglou.
Abada shot over twice as Celtic broke but United threatened in between when Niskanen curled just wide.
Soro took time to get up to speed, but Celtic began to push United back for longer spells.
United defended their box well but there were openings. The unmarked Ajeti missed what looked another good chance when he failed to get clean contact on Abada's cross. Jota volleyed just over under pressure and Rogic headed against the bar from eight yards.
Louis Appere forced a good stop from Joe Hart as United threatened a late winner before Jota hit a looping strike off the bar ahead of the full time whistle, which was met by loud jeers.
Celtic host Leverkusen on Thursday night in their second Europa League group game; kick-off at 8pm. Dundee United are back at Tannadice next Saturday when Ross County visit; kick-off at 3pm.