Barcelona failed to score in a Champions League group stage game for the first time in five years as Olympiakos held them to a goalless draw.
Victory in Greece would have booked Barca's passage to the knock-out phase but the three points proved elusive on a night when Luis Suarez hitting the top of the crossbar was as close as the visitors came.
The last time they could not find the net in a group-stage fixture was against Benfica in December 2012, when their qualification had already been assured, but Juventus' draw with Sporting Lisbon on Tuesday night means Barca at least remain three points clear at the top of Group D.
This was Barca boss Ernesto Valverde's return to a club he led to three league titles and two domestic cups during two spells, and the Olympiakos fans showed their appreciation pre-match with a banner near the halfway line that read, 'Ernesto, thanks for everything.'
Some of that good will may have dissipated had Nelson Semedo's early effort not been kept out by Silvio Proto, yet the visitors did little to build on that.
Olympiakos proved to be a disciplined unit that broke forward in numbers and Omar Elabdellaoui thought he was in at one point only for Samuel Umtiti to intervene.
In Lionel Messi, Barca still possessed an obvious difference maker, and on one of the few occasions he got free his weak effort from a corner had to be flicked away by Proto's leg, with the goalkeeper unsighted by Suarez.
The Uruguayan disappointingly fired wide at the near post before a mix-up at the other end almost cost Barcelona.
There appeared no apparent danger as Umtiti looked to shepherd the ball back to Marc-Andre ter Stegen, but nobody took responsibility and the goalkeeper's pass into the French defender almost resulted in an own goal.
The sheer noise inside Stadio Georgios Karaiskakis may have contributed to the miscommunication but Barca were doing little to silence the crowd.
Proto saved with his leg again following a typical Messi run and the frustration in the Barca camp was growing as Suarez reacted angrily to Panagiotis Tachtsidis' foul on Messi.
The belief the hosts could get a famous result was growing, on the pitch and in the stands, as Ter Stegen clawed away a Kostas Fortounis free kick in the second half.
Olympiakos could have been accused of being overconfident when losing the ball in their own half only for Suarez to pass behind Messi, meaning the Argentinian was off balance when he poked wide.
Suarez continued to look desperate to break the deadlock with a miscued overhead kick, and a cooler head would have served him better when he hesitated and lifted on to the top of the crossbar.
Two minutes from time Messi won a free kick 25 yards out but Proto was able to push his effort away.
The keeper was called upon one final time when Messi was denied with an effort heading straight for the bottom corner as Olympiakos held on for a well-deserved point.