Champions League holders Liverpool suffered defeat at Napoli this week, while Paris Saint-Germain laid down a marker by brushing aside Real Madrid and Juventus were held by Atletico Madrid.
Here, we look at five things we learned from the opening round of group-stage matches:
Liverpool not unbeatable
Holders Liverpool came unstuck for the second straight season at Napoli as a combination of poor finishing and sloppy defending cost Jurgen Klopp's men in a 2-0 loss at the Stadio San Paolo.
The famed front three of Mohamed Salah, Sadio Mane and Roberto Firmino were all guilty of wasting chances on Tuesday, while UEFA men's player of the year Virgil van Dijk made an uncharacteristic error to gift Fernando Llorente Napoli's late second.
It was a first defeat for Liverpool, on a club-record 14-match winning streak in the Premier League, since their 3-0 loss in last season's semi-final, first leg at Barcelona.
"It should hurt because there were opportunities for us," said Klopp, whose team lost 1-0 in Naples last year before going on to lift the trophy.
Tottenham's soft underbelly
Last season's runners-up Tottenham let a two-goal lead slip for the second time this season in a 2-2 draw at Olympiakos.
Mauricio Pochettino criticised his team's desire after a tame performance which was only enough for a point thanks to Harry Kane and Lucas Moura scoring in four first-half minutes.
"It's not about tactics, it's not about quality players, it's about the quality of preparing yourself to be ready to fight," bemoaned Pochettino, who also saw his team draw after leading 2-0 against rivals Arsenal earlier this month.
Spurs have won just three of their last 12 matches in all competitions.
Salzburg hoping to be 'new Ajax'
Austrian champions Salzburg laid down a marker in their first group-stage game since 1994 as teenage sensation Erling Braut Haaland scored a hat-trick in a 6-2 demolition of Genk.
The 19-year-old striker, who has scored 17 goals in nine matches this term, thinks his team can emulate Ajax, whose young side knocked out Real Madrid and Juventus in a surprise run to the semi-finals last season.
"Everything is possible. We all saw Ajax last year, it would be nice to be the new Ajax," the Norwegian told uefa.com.
Atalanta's attacking style exposed
Italian debutants Atalanta claimed third place in Serie A and a spot in the Champions League last season after a thrilling campaign that saw them finish as the leading goalscorers in the Italian top flight.
But their hopes of transferring that form to European club football's top competition suffered an early blow with a 4-0 thrashing at Dinamo Zagreb.
It was only a second win in 25 Champions League group-stage matches for the Croatian champions, and leaves Atalanta with plenty to do to get out of a group which also includes tournament favourites Manchester City.
Follow Emirates 24|7 on Google News.