The Spanish giants look likely to be thrown out of the cup after fielding an ineligible player on Wednesday night.
Third-tier Cadiz have confirmed they will file an official complaint against Real Madrid with the Spanish FA, almost certainly resulting in Rafael Benitez's side being thrown out of the competition.
Madrid beat Cadiz 3-1 in the first leg of their Copa del Rey tie on Wednesday night, but their opening goal was scored by Denis Cheryshev, a player who shouldn't have been on the field due to a suspension.
The 24-year-old Russian winger spent last season on loan at Villarreal and accrued three bookings in Spain's biggest cup competition.
And that earned him a one-game ban, meaning he should have sat out Wednesday night's fixture.
Instead, he not only started the game but smashed home the opening goal. Isco scored a brace after the break for a 3-1 win but it didn't matter as the home support celebrated.
The Spanish FA (RFEF) would not be able to take any action against Real Madrid if the affected team did not file an official complaint, but Cadiz president Manuel Vizcaino last night confirmed that his side will be contacting the RFEF.
"The board meeting has decided to make complaint about illegal player Cheryshev."
"I understand that Real Madrid have not realised, but we can't go against the competition on this."
"Real Madrid is the most important club in the history of football. This isn't an easy decision."
"Above all other interests though, this is what the fans want us to do."
Madrid's defence, if they choose to put one up, is likely to centre around them not being advised of Cheryshev's suspension.
Villarreal confirmed that they received a communiqué last year detailing the Russian winger's ban, but only because they requested it.
The RFEF last night told Spanish television that they are not obliged to send out advisory notices to clubs on suspensions, which would weaken an already feeble Madrid case.
But Emilio Butragueño claimed the club have a defence, stating:
"Article 41 of the disciplinary code says possible punishment does not have effect if the interested party is not notified personally."
Cadiz fans broke into party mode as the news filtered through in the first half, singing: "Benitez look at Twitter," and "we're going to win this game because Cheryshev can't play!"
Rafael Benitez attempted to right his mistake by subbing the attacker at half-time but the damage was done, with Barcelona defender Gerard Pique finding the whole farce rather amusing.
The worrying precedent for Madrid is that Osasuna were thrown out of the competition this season after fielding an ineligible player.
An expulsion from the Copa del Rey would be a significant blow for Rafa Benitez, who is desperate for silverware and already facing an uphill battle to get back into La Liga's title race.
And it's a touch of déjà vu for Rafa after his Valencia side were thrown out the cup in 2001 for fielding too many non-EU players for two minutes against minnows Novelda.
That said, if there is one club that can get out of this, it's Real Madrid.