Numancia striker Guillermo scored a brace at the Santiago Bernabeu as Real Madrid were held 2-2 in the second leg of their last-16 Copa del Rey tie, Zinedine Zidane's side progressing 5-2 on aggregate.
Dubiously awarded penalties converted by Gareth Bale and Isco helped Madrid establish a comfortable win in last week's first leg, in which Pape Maly Diamanka was sent off for Numancia.
And when Lucas Vazquez headed Dani Carvajal's cross in after just 10 minutes, the LaLiga giants looked set to win their first home game of the new year easily despite Zidane making 10 changes from Sunday's league draw at Celta Vigo.
Numancia had other ideas, however, Guillermo levelling just before the break and mere minutes after coming on as a substitute to give the second-tier side hope of a memorable cup shock.
Vazquez scored his second of the night to record his first Real Madrid brace in 109 appearances for the club, turning home from close range after Borja Mayoral unselfishly headed Marco Asensio's cross into the attacker's path to restore the hosts' lead.
But Numancia - who had Dani Calvo sent off deep into injury time - were not finished, Guillermo matched Vazquez's feat by heading home his second of the night to deepen the gloom surrounding Madrid, who lost their last home game 3-0 to Barcelona and are 16 points behind their rivals in the LaLiga table.
Asensio lashed an early effort into the side-netting but Madrid were in front inside 10 minutes.
Vazquez won both penalties against Numancia last week and he was the second-tier side's tormentor again, heading Dani Carvajal's pinpoint right-wing cross into the ground and past goalkeeper Munir Mohamedi.
Munir saved from Vazquez, while Asensio and Mayoral both wasted chances to extend Madrid's already comfortable advantage.
Unai Elgezabal headed Dani Nieto's corner over after 32 minutes but Numancia's attacking threat was seemingly minimal, before they scored out of nothing on the stroke of half-time.
Madrid were caught short at the back and Marc Mateu squared a low cross that Guillermo - on as a substitute for the injured Higinio Marin - could hardly miss from close range.
Munir made a simple stop from Dani Ceballos five minutes after the break as Madrid - thrashed 3-0 by Barcelona in their last home game - sought to get back in front.
And when Madrid regained the lead it was again Vazquez, the forward scoring his fifth goal of the season in all competitions.
Excellent football led to Asensio finding space on the left, with Mayoral nodding the ball into the middle of the box for Vazquez to turn past Munir, although he nervously checked for a raised offside flag before celebrating.
Kiko Casilla had to save from Nacho Sanchez and Pere Milla bundled wide as Numancia battled for a second equaliser, before Zidane introduced senior stars Isco and Casemiro in a bid to see the game out.
Numancia claimed a second equaliser, however, Guillermo flicking a sublime header beyond Casilla from an excellent Saul Garcia left-wing cross to give the second-tier side lifelong memories of their trip to the Bernabeu, Calvo's late dismissal for a foul on Isco in stoppage-time proving a mere footnote.