Manchester City needed a late winner from substitute Kevin De Bruyne to beat Leyton Orient 2-1 and reach the FA Cup fifth round on Saturday after the third-tier side threatened to shock the Premier League champions.
Jamie Donley's audacious long-range effort that came back off the bar and rebounded in off City goalkeeper Stefan Ortega had given Orient an early lead.
They maintained that advantage until 56 minutes when Rico Lewis' shot deflected in off Abdukodir Khusanov for his first Manchester City goal.
Pep Guardiola had to turn to his bench more than he would have liked just four days before a mammoth Champions League tie against Real Madrid.
But the introduction of De Bruyne made the difference as he scored the winner 11 minutes from time.
Guardiola warned on the eve of the match that his side would be made to suffer despite the massive disparity in resources between the clubs.
City lost to third-tier Wigan in Guardiola's second season in charge and he was not surprised by Orient's heroics.
"Yes, because I know the FA Cup," said Guardiola on why he expected such a battle. "Since 2018 against Wigan I know.
"It was a typical FA Cup game, that is why this competition is unbelievable. For the fans, how the crowd support and we knew they are strong."
Victory came at a cost for City, though, as new signing Nico Gonzalez hobbled off just 22 minutes into his debut after a £50 million ($62 million) move from Porto.
By that point, the visitors trailed as Donley spotted Ortega off his line from just over the halfway line.
His shot was inches away from perfection, but bounced back off Ortega and in as the German 'keeper scrambled back towards his goal.
Chances came and went for City to level before the break with Bernardo Silva's effort cleared off the line.
City needed some fortune to get back level. Half-time substitute Khusanov knew little about the equalizer as Lewis' shot hit off the Uzbek to wrong-foot Josh Keeley in the Orient goal.
Guardiola threw on Phil Foden and De Bruyne in desperate search of a winner and the veteran Belgian duly delivered.
"I'm disappointed with the last goal but at least it took the big boys to come on and beat us," said Orient boss Richie Wellens.
De Bruyne prodded in Jack Grealish's forward pass to spare City's blushes from the latest embarrassment of a difficult season.
Guardiola's men sit fourth in the Premier League and narrowly avoided a shock early exit from the Champions League in the group stages.
But they will need to be much improved when the might of Madrid come calling to the Etihad in the Champions League play-off round on Tuesday.
