Heavyweights Manchester United, Inter Milan and Athletic Bilbao made poor starts to their Europa League group campaigns on Thursday, losing against Feyenoord, Hapoel Beer Sheva and Sassuolo respectively.
United went down 1-0 - with coach Jose Mourinho fielding a reserve-packed side - on a late goal from Tonny Vilhena, despite a clear offside at the start of the counter-attack.
"Of course I'm unhappy. We lose the game in a moment we didn't deserve to," Mourinho said.
"The second half, the game was even. We were always in control. We were trying to win, they were trying not to lose. It is double unlucky because it was a clear offside.
"In the Europa League, we are going to be the team everyone wants to beat."
In that same Group A, minnows Zorya were held 1-1 by defiant Fenerbahce, for whom Simon Kjaer scored an injury time equalizer.
Meanwhile, 2010 European champions Inter Milan surprisingly crashed 2-0 in San Siro against Hapoel Beer Sheva, for whom Miguel Vitor and Maor Bar Buzaglo scored in the second half.
Fellow Italians Fiorentina were held 0-0 at PAOK and Roma could only manage a 1-1 at Viktoria Plzen.
Remarkably, the only Italian side to win were novices Sassuolo - who have never qualified for the European competitions before - with a 3-0 trouncing of 2012 finalists Bilbao 3-0 on second half goals from Francesco Acerbi, Gregoire Defrel and Matteo Politano.
"This was a great evening for our club. Now people will start to know about us in Europe, not only in Italy."
Meanwhile, Bilbao coach Ernesto Valverde said: "This was not the kind of start that we planned for, obviously. Sassuolo were too strong for us in the second half.
"But there are still five games to go, and I really think we will improve."
In that same Group F, Rapid Vienna edged Genk 3-2.
In other early matches on Thursday, Anderlecht beat Qabala 3-1, Mainz were held 1-1 by Saint-Etienne 1-0 and Zenit battled out a late 4-3 win at Maccabi Tel Aviv.
Astonishingly, Zenit came from 3-0 down to win with four goals in the last 14 minutes - with Luka Djordjevic hitting the winner in injury time.
Also on Thursday, Southampton thumped Sparta Prague 3-0, Ajax won 2-1 at Panathinaikos, Schalke prevailed 1-0 in Nice, Celta Vigo got a 1-1 draw at Standard Liege and Villarreal came from behind to beat Zurich 2-1.