Barcelona were held by promoted Osasuna as 16-year-old Ansu Fati became the champions’ youngest La Liga goalscorer.
Roberto Torres gave Osasuna the lead early on with a sweet volley from Brandon’s cross.
Fati came on at half-time for his second Barca appearance and headed in Carles Perez’s cross six minutes later to equalise.
Perez then set up Arthur to put Barca ahead with a 16-yard shot but Torres equalised with a late penalty.
That kick was awarded after Gerard Pique handled Oier’s cross.
“We did not play well at all, we were awful,” said boss Ernesto Valverde.
“They went ahead and we couldn’t get close to their area. Then we dominated the play in second half, we took the lead and had the game in our hands but they came back at us and levelled with that absurd penalty.
“We cannot afford to let a game like this one slip, we should have been more assertive on the pitch and kept on attacking.”
Who is Fati?
Winger Fati has been with Barcelona’s youth academy since 2012. He was born in Guinea-Bissau but moved to Spain at the age of six with his family.
He has not played for the country of his birth at any level, and Spanish media claim that Spain want to call him up to their youth teams.
Fati was the youngest player to make his Barca debut since 1941 when he came on last Sunday, with 12 minutes remaining, in a 5-2 win over Real Betis.
And this time he was brought on at a crucial point – when Barcelona were behind – and he got them level with a towering header from Perez’s cross.
At 16 years and 304 days, he is the third youngest player to ever score in La Liga – after former Malaga player Fabrice Olinga (16y, 98d) and Athletic Bilbao’s Iker Muniain (16y, 289d).
Barca’s worst start since 2008
Barcelona really missed star forward Lionel Messi, who is yet to play this season because of a calf injury, in another disjointed performance.
They lost on the opening day to Athletic Bilbao before coming from behind to beat Betis last weekend.
After their draw with Osasuna, they have collected just four points, their worst return from three games since 2008-09. However, they went on to win the league that season under Pep Guardiola.
That they needed two goals from substitutes in Fati and Arthur, neither of whom had scored for Barcelona before, and blew a lead late on after battling back will be a concern for boss Ernesto Valverde.