Tom Cannon returned to haunt Leicester City as he set a resurgent Sheffield United up for a win that piles pressure on Foxes boss Marti Cifuentes.
Former Leicester striker Cannon pounced on pandemonium in the penalty area in the second minute as his side's failed attempts to clear a cross teed the Republic of Ireland international up for a close-range finish.
And two minutes later, an effort from Callum O'Hare from outside the box that deflected off Jairo Riedewald put the visitors 2-0 up.
Sydie Peck's volley before the break meant Leicester had conceded three goals in the first half for the second time in as many games, having done the same in the loss at Southampton on Tuesday.
Boos rang around the ground at half-time but the mood was transformed in the second half as goals from Stephy Mavididi and Jordan James set up a thrilling finish.
Sheffield United resiliently hung on for what is their third successive Championship win, which has them only four places and five points adrift of Leicester in 15th spot.
The latest loss is undoubtedly another heavy blow in Cifuentes' reign but the concern runs deeper than the boss in the dugout.
The points amassed by Leicester so far this season come with a major question - will some be lost to a deduction?
It remains the great unknown for the Foxes as they fight a legal battle with the Premier League over spending breaches relating to the season they were promoted from the Championship as title winners in 2023-24.
The hearing into the alleged breaches has reportedly begun, external, although both the club and the football authorities have not publicly acknowledged it.
In the wider context of Leicester's need to get spending under control, the £10m sale of Cannon to Sheffield United in January meant a goal threat – and arguably a natural successor to legendary forward Jamie Vardy - was moved on to help the club's finances.
And with Cifuentes lamenting his side's lack of goals on Friday, Cannon's early strike against a side he helped to promotion just two seasons ago was particularly ironic.
Riedewald and Peck's goals exposed frailties that have plagued the Foxes' defence throughout the season and emphasised that the Blades under Chris Wilder are suddenly the fastest rising side in the Championship.
They had only moved outside the relegation places with their mid-week win against Portsmouth, although they are still just two points above the drop zone.
When Mavididi met an Abdul Fatawu cross at the back post to pull a goal back in the 53rd minute, it was their first effort on target - and one that offered a ray of hope.
From that point on, the hosts flooded forward in search of a way back into the game but the thumped finish from James in the 83rd minute was not enough to salvage anything as United desperately hung on.
Leicester City head coach Marti Cifuentes told BBC Radio Leicester:
"It was below any standard, far from the standards that I want to set for the club.
"It was extremely disappointing how we spoke over the last couple of days about reacting strong after what happened on Tuesday, about the importance of starting on the front foot the first 15 or 20 minutes at home, at our place, and we did just the opposite.
"It was just embarrassing to see some of the mistakes and it was below any kind of standards. It's very frustrating.
"The main thing that was in my mind [at 2-0 down after four minutes] was that this is not the reaction we had been working on to show to our fans.
"It was very important to get them on board in the first minutes, to show them that what happened on Tuesday was an accident, and again I think was very disappointing to concede another two goals from set plays. But in the first four minutes to go 2-0 down is simply unacceptable."
Marti Cifuentes: 'It was simply unacceptable'
Sheffield United boss Chris Wilder told BBC Radio Sheffield:
"It was outstanding. There were some tired legs at the end - I've told you about the schedule and being punished, possibly, more than anybody.
"To go Sunday afternoon, in an emotional game like that (against city rivals Sheffield Wednesday) at Hillsborough, to then go into Wednesday night and get the job done and then to come here.
"Look at their team-sheet, look at the players they have got, and especially what we did in the first half – we absolutely dismantled them.
"We were outstanding, even for the first 10 minutes of the second half. They scored a great goal that got them back in it and we did everything we needed to do. We had everything to lose, especially when the second one went in, and they had everything to gain.
"When they are 3-0 down, they can take big risks and it becomes an easier game for them and a tougher game for us. But the attitude of the team was to gain a big three points."