Barcelona were pegged back from two goals ahead as Real Sociedad snatched a superb and deserved Primera Division point at the Estadio de Anoeta.
Two goals in as many minutes from Xavi and Cesc Fabregas put Barca 2-0 up inside the opening quarter-hour and they appeared to be cruising.
But Imanol Agirretxe's header changed all that and Antoine Griezmann netted a bizarre equaliser - making good on his pre-match promise to prove his commitment to the club after the collapse of his longed-for summer move to Atletico Madrid.
The opener came when Fabregas' lofted through-ball sent Alexis Sanchez and Xavi through on goal, the former squaring the ball for the Spain midfielder to tap in.
Barcelona attempted to produce a carbon-copy goal a minute later as Fabregas ran on to Xavi's pass and squared for Pedro and, though the forward's shot was blocked, Fabregas followed up to score.
Sociedad were rightly denied a penalty after Asier Illarramendi's shot was blocked by the shoulder of Andreu Fontas as he turned away.
Sanchez was forced off by injury shortly after the half-hour mark and replaced by David Villa.
Illarramendi headed Griezmann's free-kick over before Barca threatened again, Pedro's lob cleared away from goal by defender Alberto De la Bella.
Villa and Pedro mishit shots early in the second half after cutting in off their respective wings.
And Sociedad halved the deficit when Xabi Prieto's cross from the right was headed in by Agirretxe at the far post - and astonishingly, it was 2-2 moments later in farcical fashion.
Villa's dreadful pass from inside the Sociedad half carved his own team asunder and sent Agirretxe through on Victor Valdes, who inexplicably back-pedalled. The forward's lob was pushed on to the bar by the hand of Sergio Busquets, but Griezmann followed up to head into the empty net.
Confusion followed as Sociedad seemed unsure whether the goal had been awarded, while David Zurutuza was booked after seeking a red card for Busquets. Both teams eventually lined up for the restart - though not before Barca sent on Lionel Messi.
Barca's usual composure had been rattled, with Messi scuffing wide from 18 yards and Pedro blazing wildly over after Dani Alves' cut-back.
Keeper Claudio Bravo denied Messi from a narrow angle and dived to claw away Alves' dangerous cross, but Sociedad were not finished at the other end.
Alves had to dive to head Carlos Vela's dangerous cross just wide of his own goal, and Inigo Martinez's header from the resulting corner was deflected wide.
Messi was booked for diving in the penalty area in stoppage time as Barca had to settle for a draw.
