Chelsea threw away a two-goal lead to draw with Juventus, Manchester United were far from convincing in their victory over Galatasaray and Lionel Messi rescued Barcelona against Spartak Moscow in the UEFA Champions League.
Celtic took a point against Benfica on their return to the group stages and, elsewhere, there were wins for Bayern Munich, Shakhtar Donetsk, BATE Borisov and Cluj.
Defending champions Chelsea blew a two-goal lead at the beginning of the defence of their title as Juventus spoilt Oscar's night in a 2-2 draw at Stamford Bridge in Group E.
The Brazilian was making his full debut since joining the Blues this summer and marked it in memorable fashion with two goals in two first-half minutes.
Oscar's opener, in the 31st minute, was with the help of a deflection but there was nothing fortunate about his second as a superb touch set up a stunning, curling 25-yard effort.
Juventus, though, were back in the game before half-time after Arturo Vidal finished from close range in the 38th minute.
And Chelsea's disappointment was compounded in the 80th minute, when Fabio Quagliarella took advantage of John Terry's attempt to play off side to equalise.
Manchester United's Group H campaign began in shaky fashion but Sir Alex Ferguson's men took the points in a 1-0 victory over Galatasaray at Old Trafford.
A one-two with Shinji Kagawa allowed Michael Carrick to give United a dream start, as he rounded the goalkeeper to score the opener in just the seventh minute.
United, though, lost their rhythm and it became a scrappy performance, which was epitomised when Nani's extremely poor second-half penalty was saved by Fernando Muslera.
Messi scored his 52nd and 53rd goals in the Champions League to save Barcelona against Spartak Moscow when turning what had looked like a defeat into a 3-2 win.
Seconds after losing Gerard Pique to injury, Barcelona, Champions League winners in three of the last seven seasons, took the lead through a wonderfully worked 14th minute finish from the highly-rated Cristian Tello.
However, Dani Alvez's own goal, when the full-back made hash of an attempted clearance, on the half-hour mark gave Spartak a surprise equaliser in their Group G opener.
But if the equaliser was a shock, Spartak taking the lead in the 58th minute was completely against the script as Borges Romulo, who was celebrating his 22nd birthday, finished at the end of a swift counter-attack.
Messi, though, came to life after a previously quiet night and equalised in the 71st minute before scoring the winner with 10 minutes remaining.
Celtic's return to the group stages after a four-year absence ended in a 0-0 stalemate with Benfica in Group G's other match.
Boss Neil Lennon had set his side up to ensure they did not lose and, despite a few nervy moments, the Scottish title holders achieved their target.
With Barcelona and Spartak still to come in a tricky section, Celtic will be relying on their home form to keep alive their hopes of reaching the knockout stages.
Bayern got back to winning ways in the Champions League with a seventh victory in eight Champions League outings at the Allianz Arena in a 2-1 success over Valencia in Group F.
Seven minutes before half-time, last season's defeated finalists, Bayern, were in front through Bastian Schweinsteiger before Toni Kroos made it 2-0 in the 76th minute.
Nelson Valdez pulled one back for Valencia late one but the Spanish side had Adil Rami sent off for a second booking before the final whistle.
Nordsjaelland's debut in the Champions League group stages ended in frustrating fashion as they suffered a 2-0 defeat at Shakhtar Donetsk.
It took until the 44th minute for Shakhtar to take the lead, when Henrikh Mkhitaryan made it 1-0.
The midfielder scored a second at the Donbass Arena in the 76th minute to ensure Shakhtar won just a second match in nine Champions League outings to sit above Chelsea and Juventus in Group E.
BATE Borisov won their first ever game in the Champions League group stages as they recorded a 3-1 win at Lille in Group F.
The Belarusian outfit took a sixth minute lead through a great strike from Alexander Volodko and, 14 minutes later, found themselves two goals in front through Vitaly Rodionov.
Two minutes before half-time, BATE were in dream land as Edgar Olekhnovich made it 3-0 and there was no way back for Lille, despite Aurelien Chedjou's 60th minute consolation.
Cluj recorded one of the surprise results of the night as they stunned Braga in United's Group H with a 2-0 win.
Rafael Bastos gave the Romanian champions the lead in the 19th minute and the former Braga player then made it two in the 34th minute with a well-taken effort.