Lionel Messi preserved Barcelona's unbeaten start to the Champions League by getting the tying goal in a 1-1 draw at AC Milan while Arsenal's perfect record ended with a 2-1 loss at home to Borussia Dortmund on Tuesday.
Atletico Madrid made it three wins in three matches thanks to two goals from striker Diego Costa in a 3-0 victory at Austria Vienna.
Chelsea claimed the performance of the night by beating Schalke 3-0, with striker Fernando Torres also scoring twice to put the 2012 champions on top of Group E.
Back from a thigh injury, Messi scored from close range to match Robinho's opener for Milan and keep Barca in charge of Group H on seven points, two ahead of the Italians.
Robert Lewandowski grabbed an 82nd-minute winner for Dortmund, last season's finalist, at Arsenal in Group F. They are joined in a three-way tie with six points by Napoli, which won 2-1 at Marseille to send the French side to a third straight loss.
Celtic and Zenit St. Petersburg got their first wins while Basel drew 1-1 at Steaua Bucharest.
Milan and Barca were meeting in Europe's top competition for the seventh time in three seasons.
There was no shortage of stars at the San Siro and two of them combined for Milan's opener, with Robinho exchanging passes with Kaka before slotting past Victor Valdes.
However, Milan has never been able to handle Messi — the Argentina forward has now scored six times against the Italians — and he equalized after collecting Andres Iniesta's pass, evading Kevin Constant and shooting into the bottom corner.
The teams meet again in two weeks, with Barcelona knowing a victory would give them 10 points and close to the knockout stage.
Celtic is two points behind Milan after beating Ajax 2-1 thanks to goals by James Forrest and Beram Kayal in a 10-minute spell either side of halftime.
Ajax got its goal through Lasse Schone in second-half stoppage time, by which time Celtic had been reduced to 10 men after Nir Biton was red-carded in the 87th minute.
Arsenal's first loss since mid-August, on the night of manager Arsene Wenger's 64th birthday, was sealed by Lewandowsi's unforgiving finish with eight minutes left.
Back in London for the first time since losing last season's final to Bayern Munich in May, Dortmund went ahead through Henrikh Mkhitaryan in the 16th minute only for Olivier Giroud to get the tying goal just before the break.
José Callejón and substitute Duván Zapata were the scorers for Napoli at the Stade Velodrome, with Andre Ayew netting for Marseille four minutes from the end.
Atletico, tied with Barca at the top of the Spanish league, is the only team in the four groups who played on Tuesday to have three straight wins after Costa added to the opener scored by Raul Garcia in the Austrian capital.
Diego Simeone's side is five points clear of Zenit, which beat FC Porto 1-0 thanks to Aleksandr Kerzhakov's goal with five minutes left.
Porto had done well to nearly hold out for a draw after seeing Hector Herrera sent off for two yellow cards after just six minutes.
In Group E, Torres marked his 100th start for Chelsea by scoring in the fifth and 69th minutes, with Eden Hazard completing an unexpectedly easy win against a Schalke team that led the group at kickoff.
Chelsea is ahead of Schalke on goal difference, with Basel two points back after conceding a late goal by Leandro Tatu to draw at Steaua, which claimed a first point in group play.