FA Cup holders Leicester were dumped out of the competition in the fourth round after being thrashed 4-1 by an inspired Nottingham Forest at the City Ground.
The Championship side were unfazed by the arrival of their Premier League opponents and took the lead after 23 minutes when Philip Zinckernagel turned home from inside the six-yard box after Kienan Davis nodded down Brennan Johnson's floated cross from the right.
Forest fans barely had time to celebrate their side's goal before they were on their feet again just 87 seconds later, with Johnson cleverly slotting through the legs of Danny Ward after Daniel Amartey's dreadful backpass played him in.
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During the celebrations, a supporter ran onto the field and appeared to throw punches towards Forest players.
Leicester apologised to Forest after the game and pledged a lifetime ban to the fan in question, with a spokesperson saying: "The club is appalled by such behaviour and offers its apologies to Nottingham Forest and its players whose safety was compromised.
"We applaud the swift action taken by City Ground stewards in apprehending the individual involved, who will be subject to a lifetime ban from all Leicester City fixtures at home and away.
"We will offer every support to the relevant authorities in pursuing further appropriate action."
Nottinghamshire Police confirmed on Sunday an arrest had been made, with chief inspector Neil Williams saying: "We have arrested a man in connection with an incident that took place today (Sunday 6 February) during the first half of the Nottingham Forest and Leicester City FA Cup match.
"Investigations are in the very early stages and our enquiries into the circumstances surrounding the incident are ongoing."
The incident didn't derail Steve Cooper's side, who were celebrating again just after the half-hour mark when some poor set-piece defending allowed returning captain Joe Worrall to steer a header past Ward and hand Forest a stunning 3-0 lead.
Leicester were handed a lifeline when Kelechi Iheanacho (40) beat Forest goalkeeper Brice Samba - who had rashly raced out of his area - to a James Maddison through ball and rolled the ball into the empty net.
Brendan Rodgers made a change at half-time, replacing Harvey Barnes with Patson Daka, but it failed to help Leicester maintain any momentum that the goal may have generated.
It was Forest who began the second period on top, with Zinckernagel - whose performance may have new Watford manager Roy Hodgson wishing the forward was available to help his parent club's battle against relegation - driving at the Leicester defence before teeing up Johnson for a shot that was deflected wide.
Maddison went close for the Foxes when he bent a shot wide of Samba's far post after 52 minutes, but they found themselves 4-1 down after an hour when the impressive Djed Spence cut inside from the right, exchanged passes with Zinckernagel and drilled a finish past the exposed Ward.
That goal ended the game as a contest, with several Leicester players - including Maddison, who picked up a booking for a needless foul late on - becoming visibly frustrated.
Forest could have made it 5-1 when Max Lowe steered a left-footed effort from the edge of the box just wide of Ward's far post after 66 minutes.
But while Forest continued to threaten, Leicester failed to create any chances of note as they watched their hold on the FA Cup slip away in highly disappointing fashion.
Amartey tried to make amends for his earlier error with a fizzing left-footed volley from outside the box that Samba held smartly down to his right, and Ricardo Pereira sent a low shot just wide after dribbling into the area, but Forest were relatively untroubled as they backed up their third-round win over Arsenal with another spectacular victory against top-flight opponents.
As for Leicester, they exited the tournament with a whimper at the hands of their East Midlands rivals and now have just five wins from their last 18 matches from all competitions.
Cooper: We could have scored more
Nottingham Forest were bottom of the Championship when Steve Cooper replaced Chris Hughton in September, but the manager has sparked an incredible revival that has seen the club rise to eighth, just two points outside the play-offs.
The back-to-back FA Cup wins over Premier League opponents make Cooper's work at the City Ground even more impressive, and he said: "It was a good day, for sure. A great result obviously, but a performance to match.
"I thought first half we were excellent from an attacking point of view, obviously scoring three and creating one or two other chances. Dare I say it, we could have scored more in that first half.
"A little bit disappointing to give them one back, but I like the fact it didn't upset our rhythm or momentum. You can easily lose a bit of focus when things like that happen, but we didn't, so that's a good sign of mentality and concentration.
"Second half was always going to be a little bit of a different game in terms of us being a bit deeper, but stuck to the plan, minimised any real threat from them and looked a threat ourselves.
"Just a really pleasing day - one for the supporters. We get attendances like this all the time and they really get behind us, so days like today really help the connection."
Johnson: I never wanted to leave Forest
Brennan Johnson's outstanding performance - including his third goal in his last four matches - demonstrated why Brentford wanted to make him their record signing last month.
But the 20-year-old insisted he was keen to remain at his boyhood club during the transfer window, telling BBC Sport after the game: "I love this club and I always wanted to stay, the whole window.
"Once January the 31st finished I was just focused on finishing the season here."
Rodgers: Fan incident unacceptable
Leicester's disappointment at their FA Cup exit was compounded by the incident involving the supporter running on to the pitch, with Foxes boss Brendan Rodgers saying he could "never accept" such behaviour.
Rodgers said: "There's no room for it and I'm sure as a club we'll deal with that. It should never happen, no matter how disappointing your team is and how they've been playing.
"The players have to be safe on the field. It was a great atmosphere today. I feel a big disappointment for our supporters, but we can never accept that behaviour."
FA Cup fifth round draw in full
- Luton Town vs Chelsea
- Crystal Palace vs Stoke City
- Peterborough United vs Manchester City
- Liverpool or Cardiff vs Norwich
- Southampton vs West Ham
- Middlesbrough vs Tottenham Hotspur
- Nottingham Forest vs Huddersfield Town
- Everton vs Bournemouth or Boreham Wood
The ties will be played in the week commencing Monday February 28, 2022.
What's next?
Nottingham Forest travel to face Blackburn in the Championship on Wednesday at 7.45pm, live on Sky Sports Arena. Liverpool host Leicester City in the Premier League on Thursday at Anfield at 7.45pm.