Real Madrid were thrashed 4-1 at struggling Valencia on Sunday as Carlos Soler scored a hat-trick of penalties to raise further doubts over La Liga's reigning champions.
Madrid took the lead through Karim Benzema but presented Valencia with four gifts at Mestalla, including three spot-kicks and an own-goal scored by Raphael Varane, to fall to another surprise defeat.
"There is no justification, no excuses," said Zinedine Zidane. "It's a bad game and a bad day."
This is the first time Madrid have conceded four goals since letting in five against Barcelona in October 2018 and puts the brakes on their recovery that had followed a rocky start to the season.
After back-to-back losses to Cadiz and Shakhtar Donestsk last month, Zinedine Zidane's side appeared to have turned a corner with wins over Barca, Huesca and Inter Milan.
Yet this result puts the pressure back on ahead of the international break, with Madrid now fourth in the table, four points behind early leaders Real Sociedad.
A disappointing night also got worse when Benzema was forced off late on with what looked like a groin injury.
"Everything happened to us," Zidane said.
Valencia, meanwhile, started the day in 17th after enduring a nightmare start to the campaign, on the back of a transfer window which saw them sell five key players and fail to replace any of them.
A stirring victory though pushes Javi Gracia's team up to ninth and may prove a turning point in their season.
There was certainly an element of fortune in Valencia's goals, all of them unusual and some of them bizarre.
But Soler did brilliantly to beat Thibaut Courtois three times, particularly having missed his first attempt, which was retaken after the Belgian came off his line.
Madrid were also without Casemiro after the Brazilian tested positive for coronavirus but a heavy loss continues a worrying trend of defensive vulnerability.
Solidity was the key to them winning the title last season but they are now seven matches without a single clean sheet.
"We have to be strong in defence," said Zidane.
"If we have this, we have quality up front to make a difference."
The first half turned on two decisions that went Valencia's way but Madrid could have few complaints.
They took the lead when Benzema unleashed from the edge of the area but conceded a penalty when Jose Luis Gaya's cross struck the arm of Lucas Vazquez.
Chaos followed as Soler's penalty was saved by Courtois and then his rebound came back off the post to Yunus Musah, who finished on the half-volley.
Replays showed Musah had entered the box too early but Courtois was also off his line, making his save illegal and handing Soler a retake.
He made no mistake at the second attempt and then Valencia pulled in front when Varane skewed a cross towards his own-goal and got tangled with Courtois while attempting to clear.
The ball was shown to have crossed the line while Valencia were lucky to escape punishment for a tackle on Marcos Asensio by Denis Cheryshev in the build-up.
Madrid had work to do but Valencia started the second half stronger, Lee Kang-In hitting the post before two more penalties were awarded.
Marcelo wrapped his foot around Maxi Gomez in the area before Sergio Ramos tackled Musah and as the ball popped up, Madrid's captain knocked it away with his left hand.
Soler stepped up for this third and fourth spot-kicks of the match and scored both to compound the defending champions' misery.
Real Sociedad regained top spot after winning 2-0 against a depleted Granada, who could face punishment for fielding fewer than the required number of first-team players.
Nacho Monreal scored in the 22nd minute and Mikel Oyarzabal converted a penalty in the 27th at Anoeta to send la Real back above Atletico Madrid, who had briefly taken first place after beating Cadiz on Saturday.
Atletico are three points behind but have two games in hand.
Granada, who drop to fifth, had only seven first-team players available and finished with only four on the pitch, one fewer than La Liga's minimum of five, with the rest made up from the youth team.
It remains to be seen if La Liga take action, having already rejected Granada's request on Saturday for the game to be postponed.
Unai Emery's Villarreal move up to second in the table after they won 3-1 against Getafe.