Celta Vigo scored twice in three second-half minutes to come from behind, beat Celtic and leave the Scottish champions' Europa League hopes hanging by a thread.
Javi Rodriguez first headed in a free-kick from close range, then lashed in a stunning second to leave Martin O'Neill's side pointless after three matches following defeats by Ferencvaros and Benfica.
Spanish side Celta enjoyed spells when they dominated the ball in the first half without really opening Celtic up.
Ferran Jutgla fired an effort just wide and also forced Sam Johnstone into a smart save just before the break.
The Celtic fans were screaming for a penalty when Callum Osmond went down on the edge of the box from a push from Javi Rueda.
Replays showed the incident was just outside the penalty area, but the referee saw no infringement at all and waved play on.
The Scottish Premiership side had made little impression in the final third, but barely a minute into the second half they took the lead.
Haissem Hassan had had little change out of Marcos Alonso, but he ghosted by the former Chelsea man and delivered a cross into the box. It was only half-cleared to Mika Baur, who swept home a smart side-foot finish low past Ionut Radu.
Liam Scales saw a header hit the crossbar, but Celta turned the game on its head with two goals in three minutes.
Rodriguez met a terrific free-kick delivery with a bullet header to level things up, and then unleashed a rocket from 20 yards which flew past Johnstone and into the net for what proved to be the winning goal.