Celtic were punished again for their defensive errors as they followed up their Old Firm loss with a 3-1 home defeat to Ferencvaros in their Europa League opener.
Martin O'Neill vowed his team would get things right after their League Cup collapse against Rangers, but it was more of the same after a promising start against the Hungarians.
They had plenty of early possession, but were soon a goal down when Liam Scales turned Bamidele Yusuf's shot into his own net after Kieran Tierney had failed to deal with a cross.
With half-time approaching, Mika Baur slammed the ball into the net to pull Celtic level only for the visitors to quickly restore their lead.
Yusuf beat Tierney and Callum McGregor was too slow to clear the ball, leaving Kristoffer Zachariassen to flick the ball past the returning Viljami Sinisalo.
The mistakes continued in the second half. Colby Donovan, handed his first Scotland call-up this week, was caught out and Australian winger Daniel Arzani smashed the ball into the top corner.
Celtic did have a flurry of late chances, but they could not salvage anything from the game as the fans who stayed until the full-time whistle made their frustrations clear.
It is back-to-back defeats for O'Neill's side ahead of Sunday's Premiership clash with Rangers, live on Your Site.
8: CHANCE! James Forrest crosses the ball into Camilo Duran. He takes a double touch and it's past the post.
20: GOAL FERENCVAROS! Tierney fails to deal with a cross and it falls to Yusuf. His shot takes a final touch off Scales before it hits the back of the net.
23: SAVE! Arzani gets past Cameron Carter-Vickers, but Sinisalo gets down to push the ball wide.
31: CHANCE! Haissem Hassan tries to meet Forrest's cross at the back post, but Cadu is there to clear.
32: CLOSE! Hassan sends the ball to McGregor on the edge of the box, but his strike is deflected past for a corner.
44: GOAL CELTIC! What a time to equalise! Tierney sends the cross into Baur to slam into the net.
45+1: GOAL FERENCVAROS! Nightmare from Celtic! Yusuf beats Tierney and McGregor before setting up Zachariassen to flick the ball past Sinisalo.
47: GOAL FERENCVAROS! Celtic lose possession far too easily and Arzani punishes their latest mistake with a brilliant strike into the top corner.
67: CHANCE! Carter-Vickers sends the ball to Duran to cross in for Kasper Hogh, but it just evades the Celtic sub.
68: WIDE! Benjamin Nygren sends a curling effort just past the post.
72: SAVE! Sebastian Tounekti's powerful strike is pushed clear by Adam Varga.
73: CHANCE! With Varga well off his line, Nygren strikes from distance but it is over the bar.
Your Site' Alison Conroy:
Celtic started well, and Forrest appeared to be rolling back the years as he caused problems for Ferencvaros down the left.
But their defensive frailties soon proved costly, again. The goals were self-inflicted.
How to solve the problems at the back is the big question, but there is no obvious answer.
O'Neill made four changes from Sunday's defeat to Rangers but they did not appear to make any difference.
Celtic's big summer signings like Duran and Hogh have yet to step up in the big games this season and, for a frustrated Hoops support, that is a real worry.
O'Neill admits he is still trying to work out his best starting XI, but he does not have too much time to figure that out.
No matter what happens on Sunday, they will be top of the Premiership heading into the international break. But they need a win - two Old Firm defeats and a Europa League loss in eight days would not be accepted by the fans.
Just months after snatching a league and cup double, is the pressure now on O'Neill?
Celtic manager Martin O'Neill:
"We're naturally disappointed getting beaten.
"It's not the start we wanted. We have seven games in which to try and get some points on the board. Really need to concentrate now on Sunday [against Rangers].
"Tonight was a European lesson for us in many aspects. We'll get ourselves together. We need to re-energise ourselves as much as anything else.
"When we have decent possession of the ball, just don't give it away. But also I think taking a few risks as well too - risks in the proper areas of the field."
On the full-time booing, O'Neill added: "I've been here 25 years ago where the crowd can react like that, so it's genuinely nothing new to me.
"I think that they were showing a lot of frustration as well. At times when we had the ball, we looked as if we were going forward and the next thing you know we've put two passes backwards.
"So those things definitely frustrate you and that's something that we'll have to try and iron out."
