Thibaut Courtois' blunder helped Alaves claim a 2-1 win over Real Madrid at the Alfredo Di Stefano stadium.
Alaves, who had won just two LaLiga matches this season coming into the game but look a side on the up, took an early lead through a penalty from Lucas Perez.
Los Blancos had recovered from the loss of Eden Hazard to another injury problem after only 30 minutes to come back into the game – only for a costly error from Courtois to gift the visitors another goal.
Just a couple of minutes into the second half, the Belgium international knocked a backpass straight to Joselu, who slotted the former Chelsea goalkeeper's stray ball into an empty net.
Despite a late consolation from Casemiro, there was no way back for Zinedine Zidane's side, who now six points behind joint leaders Real Sociedad and Atletico Madrid after 10 games.
Both teams paid their respects to Diego Maradona before kick-off, with Alaves, hoping to build on a four-game unbeaten run that included draws against Barcelona and Valencia, soon taking the lead from the penalty spot.
Victor Laguardia headed the ball down from a corner, with the referee signalling for handball against Real defender Nacho.
Perez struck the penalty past Courtois to put the visitors in front after just four minutes.
Los Blancos, who won 2-0 at Inter Milan in the Champions League on Wednesday night, slowly regrouped, but Alaves remained a threat on the counter-attack.
Belgium midfielder Hazard was forced off with what looked like another injury problem after half-an-hour, replaced by Rodrygo.
Toni Kroos broke into the Alaves box, but goalkeeper Fernando Pacheco made a save with his feet before showing great reflexes to also stop the follow-up from the Germany midfielder.
Mariano headed a cross from Luka Modric over as Real were unable to find a way back into the match before half-time.
The reigning Spanish champions fell further behind just five minutes after the restart following a blunder from Courtois.
The 28-year-old collected a bouncing back-pass from Marcelo, took a touch – but then contrived to knock the ball straight out to Joselu, who sent his shot back into an unguarded net.
Real boss Zidane made a change on the hour, replacing Marco Asensio with Vinicius Junior.
Courtois then produced a smart stop to deny Alaves a third as Perez ran clean through on goal.
With 20 minutes left, Zidane hauled off Marcelo, Kroos and Modric to send on Isco, Ferland Mendy and Martin Odegaard.
Rodrygo dragged his shot wide, before Mariano's header was cleared off the line.
With five minutes left, Casemiro knocked the ball in from close range after a corner, but there was to be no late drama.
