Celtic threw away a two-goal lead and lost 4-3 to Real Betis in a pulse-pounding start to the Europa League group stage, as West Ham rode to a 2-0 win over Dinamo Zagreb.
Eighteen years after reaching the UEFA Cup final in Seville, Celtic returned to Andalusia and got off to a dream start at the Benito Villamarin.
Albian Ajeti bundled the opener thanks to a VAR review after the goal was initially disallowed for handball.
The Swiss striker was then knocked down in the penalty area by Claudio Bravo and Josip Juranovic buried his first Celtic goal from the penalty spot.
But the defensive weaknesses that characterized Ange Postecoglou’s early days as leader of the Glasgow giants were exposed to cost the visitors a crucial victory.
Two goals in three minutes wiped out Celtic’s lead as Juan Miranda drove in the goal before Borja Iglesias selflessly remembered Juanmi to equalize.
Iglesias completed the comeback with a near-post flick early in the second half before Juanmi’s shot flew into the inside of the penalty area.
Anthony Ralston withdrew a goal three minutes from time to give Celtic some hope, but Betis held onto all three points.
Bayer Leverkusen was also the winner in Group G as the Germans came from behind to beat Ferencvaros 2-1 thanks to a second half winner from 18-year-old star Florian Wirtz.
West Ham enjoyed a much more comfortable return to European football in the Croatian capital.
Michail Antonio picked up a weak back pass to continue his early season goalscoring streak with his fifth goal in as many games.
Declan Rice then galloped from his own half before producing a powerful end five minutes into the second half.
Genk was the winner in the other match in the group of West Ham with 1-0 at Rapid Vienna.
Galatasaray sparked a blockbuster clash with Lazio 1-0 thanks to an own goal from Thomas Strakosha.
In the same group, Marseille was held 1-1 by Lokomotiv Moscow when former Chelsea midfielder Tino Anjorin scored a late equalizer for the Russians.
Leicester host Napoli and Rangers face Lyon in the 1900GMT kick-off roster.