Trust Sven-Goran Eriksson to prepare a cold welcome for an old flame. The former City manager gave his exes a real hard time on their first meeting since the split in 2008, and City were probably lucky to get another date.
Goals from James Milner and Carlos Tevez at the Walkers Stadium might normally have been enough to ensure passage straight through for a trip to Notts County, but City's defending - and a shock error from goalkeeper Joe Hart - meant a replay will be necessary.
It is one rendezvous with his old mentor that City boss Roberto Mancini could well have done without, given that the Eastlands re-match will add to the burden as the Blues boss looks towards securing a Champions League place and beyond.
As if he needed any more incentive to beat his former club, now managed by his former protege and the man who describes him as "a brother", Eriksson knew before kick-off that a victory against the odds would book a fourth-round tie with another of his old clubs.
It soon became clear that Leicester's boss, Mr Nice Guy off the pitch but deadly serious about the game and eyeing a play-off place for a side still improving, had told his team to enjoy the occasion and not be afraid to attack their more illustrious opponents.
City's defenders often lacked that watertight bond that has helped propel the team to second place in the table, and they looked askance at each other as Leicester stunned their visitors with a set-piece goal timed at just 45 seconds. It gave the tie some real bite but should certainly have been prevented.
Leicester's short corner routine saw the cross directed in along a flight path that invited more than one Blues defender to clear. Nobody did, and Ivory Coast international Sol Bamba, making his debut for the home side, had a simple close-range finish to make his mark.
Conceding such an early goal need not be a disaster, though. City buckled down and refused to panic, and might have equalised soon after when Patrick Vieira met Johnson's corner with a sidefooted volley that the keeper saved with his legs.
But Mancini's men did not have to wait long to pull level, and it was a timely first City goal for Milner that hauled the Blues back on terms after 22 minutes. The summer signing exploited Carlos Tevez's pass to the full, beating two defenders then cracking a low left-footer into the bottom corner.
Milner's maiden effort, half a season after his move from Aston Villa, came just in time for the travelling army's "Poznan" celebration in tribute to Neil Young, scorer of City's winner against Leicester in the 1969 FA Cup Final.
Bamba was posing problems for international team-mate Kolo Toure and Joleon Lescott, but there were more celebrations to come for Blues supporters even though Johnson glanced a Jerome Boateng cross wide of goal with Jo poised at the far post for a simple tap-in. No matter.
Captain Tevez had not been particularly influential before now, but almost on the stroke of half-time the Argentine was perfectly positioned at the near post to grab the lead for City from a short corner
It was his 13th goal of the season but his first outside the Premier League, and his timing was impeccable.
Mancini opted to shore things up for the second half, with Nigel de Jong replacing Shaun Wright-Phillips, who had found few opportunities to shine before the interval.
Leicester clearly felt that they were still in the tie, and so it proved when Paul Gallagher pinged in a cross that should have been no problem for Hart. But the England goalkeeper, perhaps deceived by the ball's bounce, fumbled his low catch and King was there to bundle the ball home before Hart could recover.
It could have been worse soon after, but when Bamba soared to meet yet another poorly-defended corner, Milner was on the far post to clear the well-placed header that threatened to put Leicester in front.
When City did get the chance to avoid an unwanted replay, Tevez's swivel and pass in the penalty area put De Jong of all people in a prime position to score. The midfielder, deep in unfamiliar territory, stroked the ball straight towards the grateful goalkeeper.