Real Madrid's final Champions League rehearsal was running perfectly on Saturday until Cristiano Ronaldo was substituted and Villarreal scored twice to snatch a 2-2 draw.
Gareth Bale and Ronaldo put Real in cruise control at the Estadio de la Ceramica but the Portugese, along with Luka Modric, was taken off shortly after the hour and Villarreal came surging back.
It was two substitutes that did it, Roger Martinez unleashing a stunning strike into the top corner before SamuCastillejo struck the equaliser five minutes from time.
Castillejo took advantage of some hesitation from Luca Zidane, Zinedine Zidane's 20-year-old son, who was making his debut for Real in goal.
"I am happy for his debut, it is an important day for him and his coach," Zidane senior said afterwards.
"For his father, we will see what he says at home."
The draw means any hopes Real had of snatching La Liga's runner-up spot off city rivals Atletico are ended but their attention will quickly turn to Liverpool and next weekend's showdown in Kiev. Villarreal jump above Real Betis into fifth.
If Liverpool boss Jurgen Klopp had been spying for signs of weakness he would have found few in the opening hour, when Real looked slick, clinical and set for a comfortable victory.
But, defensive frailties were exposed again and perhaps this result underlines that if Ronaldo can be neutralised, this is still a team that is far from unbeatable.
"We are ready," Zidane said. "We have not had any injuries today and we are ready to play our last match and our final."
Ronaldo was withdrawn after 61 minutes, the 33-year-old showing no sign of discomfort in what was his first outing since spraining his ankle against Barcelona a fortnight ago.
Bale too will hope his smart turn and finish could yet earn him a starting spot against Liverpool, even if Zidane has shown propensity for ignoring the Welshman's league form in the past.
After scoring twice against Celta Vigo last weekend, Bale was excellent again here and he at least heads to Ukraine knowing he could not have done much more. He has now scored 17 goals in 19 matches this year when playing more than 60 minutes.
"Gareth has never lowered his level," Zidane said. "I am happy with his role, for what he does, he has not changed anything."
- Zidane names strong side -
Liverpool will have played only one match in 20 days by next weekend and Zidane opted for rhythm over rest, picking a strong team that could be close to his line-up in seven days' time.
The Frenchman, however, has tended towards caution in Europe and it would be a surprise if Bale and Isco both started ahead of Lucas Vazquez, and even Marco Asensio, next weekend.
There was a debut in goal for Zidane's son Luca, who joined Real's academy when Zidane was playing for the club in 2004 and has been a regular presence in the squad this season.
Luca was born in May 1998, a month before his father inspired France to win the World Cup.
Bale opened the scoring in the 10th minute with a goal largely of his own making as he fooled Daniele Bonera by running over Modric's pass, before driving into the bottom corner.
Ronaldo doubled the advantage before half-time, nodding home a nonchalant Marcelo cross from the outside of the Brazilian's foot as Real strolled into the interval.
But Ronaldo and Modric trundled off and the game turned on a brilliant shot from Martinez, who jinked inside from the left and found the far top corner.
If Luca Zidane could have done little to stop that one, he might have done better with the equaliser as he rushed out to intercept a clipped ball over the top but failed to claim.
Castillejo touched it round his flailing opponent and clipped a sharp finish into the empty net.
Earlier in the day, Sevilla maintained their strong end to the season with a 1-0 win over Alaves. They finish seventh, below Real Betis, who slipped to a surprise 3-2 defeat at Leganes.