Barcelona lost ground on La Liga leaders Real Madrid as they slumped to a skock 2-1 home defeat to newly-promoted Deportivo Alaves.
In the first league encounter between the two sides since 2005-06, Mauricio Pellegrino's side were fantastic throughout, defending stoically before Deyverson's close-range strike put them ahead prior to half-time.
Barca finally made their domination pay after the interval, though, Jeremy Mathieu heading home the leveller after meeting Neymar's precise corner.
Mathieu squandered a golden opportunity moments later and, after having a goal ruled out for offside, Alaves restored their advantage when former Athletic Bilbao winger Gomez took advantage of slack Barca defending to drill home following a fantastic run.
With Real Madrid having made it three wins from three with victory over Osasuna earlier on Saturday, Luis Enrique's side piled on the pressure, but Alaves held firm to secure a famous victory and extend their unbeaten start to the season.
Barca were boosted by the return of Neymar after the Brazil forward led his nation to Olympic gold, though – most likely with an eye towards Tuesday's Champions League tie with Celtic – Luis Enrique elected to rest both Lionel Messi and Luis Suarez, instead handing a debut to Paco Alcacer.
But it was the visitors who were the first to have an attempt on goal, Jasper Cillessen, making his debut in the absence of Marc-Andre ter Stegen, equal to a well-struck drive from Gomez.
Fernando Pacheco was called into action for the first time on the half-hour mark, making an excellent save in order to tip Aleix Vidal's rasping effort onto the crossbar following a well-worked Barca free-kick.
In spite of Barca's possession, though, it was Alaves who took a shock lead when Deyverson latched onto a terrific cross from Kiko Femenia and tapped home past Cillessen.
Barca were shocked, but almost produced an immediate response when Neymar got onto the end of looping cross, only to head wide of a gaping goal, before Ivan Rakitic fired over from close-range on the stroke of half-time.
However, Barca wasted no time in restoring parity after the restart, the unmarked Mathieu rising highest to connect with Neymar's whipped corner and heading in past Pacheco.
Mathieu should have doubled his and Barca's tally moments later, but the France defender's effort was instead an early candidate for miss of the season, as he somehow thumped wide from six-yards out.
Alaves thought they had snatched a second when Edgar Mendes volleyed home, only for the strike to be correctly disallowed for offside, before Alcacer flashed an effort wide at the other end.
Luis Enrique decided to bring on the cavalry after the hour-mark, introducing both Messi and Andres Iniesta into the fray.
But they had little time on the pitch before Barca found themselves behind once more, Gomez weaving his way into the area before despatching a low finish into the bottom corner.
And despite a late onslaught, including Messi's agonising injury-time miss, Barca could not muster a comeback as they slumped to a shock defeat, their first to Alaves since February 2000, and lost early ground on league-leaders Madrid.