The Championship side, who knocked out Arsenal in the previous round, once again went into the game as underdogs.
But they blew away the Foxes in a pulsating 10-minute spell in the first half.
Two goals in a minute, coming from Philip Zinckernagel and Brennan Johnson, gave Forest a two-goal lead after 23 minutes played.
They scored again 10 minutes later when Joe Worrall headed home. As Forest’s players were celebrating the goal, they were attacked by a Leicester City ‘fan’.
The individual in the away end managed to get on the pitch and inexplicably threw a couple of punches at Forest’s players.
Security guards made their way on to the pitch and, fortunately, were able to stop the individual before any damage was done.
The individual was thrown to the ground and then taken from the field.
The moment was captured on fan footage and you can view it from two angles below…
Never change Leicester fans never change pic.twitter.com/BnUedtdFT3— Ciaran Durham (@ciaran7durham) February 6, 2022
The supporter involved should be ashamed of themselves. He has been arrested and is now in police custody, Mark Chapman revealed during coverage of the game on BBC.
Leicester said in a statement after the game, per LeicesterMercury: “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.”
Nottingham Forest went on to win the game 4-1.
Kelechi Iheanacho pulled one back for Leicester just before half-time.
But Djed Spence’s goal 16 minutes after half-time reestablished the Championship outfit’s three-goal lead and they held on to dump the FA Cup holders out of the competition.
Nottingham Forest’s reward will be a home tie against fellow Championship side, Huddersfield Town, in the fifth round.