Manchester United blew a 3-0 lead and needed a last-gasp winner from Championship side Coventry to be overturned but scraped into the FA Cup final with a 4-2 penalty shootout victory, where they will once again meet Manchester City.
Their celebrations when Rasmus Hojlund scored the crucial penalty were fuelled by relief after an astonishing and woeful collapse had almost cost them.
Perhaps this was Erik ten Hag's 'Mark Robins moment' - the Coventry boss saved the job of Sir Alex Ferguson with a winner in this competition in 1990. Hojlund's spot-kick may have saved Ten Hag - but the fightback from Robins' side exposed all the flaws of the Dutchman's tenure.
Coventry pulled off a similar late show against Wolves in the quarter-finals. But this was a horror show from United who hit the panic button when Ellis Simms pulled one back with 19 minutes to play.
It had all been so comfortable until then, with Scott McTominay, Harry Maguire and Bruno Fernandes putting United into a seemingly unassailable position. But with a centre-back injury crisis compounding a vulnerability and openness in United's play Coventry capitalised.
Callum O'Hare's deflected shot looped over Andre Onana and in the 95th minute, Haji Wright levelled it up from the spot after Aaron Wan-Bissaka's handball.
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There was drama in extra-time with Fernandes and Simms both hitting the bar before Victor Torp thought he'd scored the clincher on 121 minutes only for it to be ruled out by VAR for offside against Wright in the build-up.
When Casemiro's opening penalty of the shootout was saved by Bradley Collins it seemed Coventry would complete the most incredible FA Cup turnaround - but O'Hare was denied by Andre Onana and when Sky Blues captain Ben Sheaf blazed over, Hojlund fired in a fine penalty to clinch it.
For 1987 FA Cup champions Coventry, 10 years on from being forced to play at Northampton's stadium, this was ultimately another shootout defeat at Wembley after last season's play-off final loss to Luton. But another reminder of how far the club has come from the dark days a decade ago.
For Ten Hag this was a victory but a chastening one. Two years to the day since it was announced he would be United head coach he has somehow steered his side to a record 22nd FA Cup final, where they will hope to exact revenge on rivals Man City on May 25. But even in victory, there will be fierce criticism.
Ten Hag: We're not embarrassed to win like that - it's a big achievement to reach another final
Man Utd boss Erik ten Hag:
"Two times in [the final in] two years, that's a good performance, with all the setbacks we've had this season. Mixed feelings is clear. But it's a huge achievement to be twice in a FA Cup final in two years.
"But of course when you're so in control and 3-0 up we should bring it over the fence. The way we did it was not good.
"But in the penalty stage how we act there, how determined we were showed good character. But from the 70th minute until the end we made mistakes and it shouldn't happen.
"We can play on very high levels but also in the same match we can go to very low levels. That's not explainable. It has to do with managing the game, taking responsibility, keeping organised and making the right decisions.
"I have to teach my players. But the most difficult thing is to put ourselves in a winning position. The second thing is easier. Today we got away with that.
"It's not an embarrassment, it's a huge achievement."
Keane: Man Utd looked like the Championship team; they rode their luck
Roy Keane on ITV Sport: "The substitutions, the goal gave Coventry belief and all of a sudden they looked like a Premier League team in extra-time and United looked like a Championship team.
"Manchester United's players and supporters didn't go over the top because they knew they were lucky today. They were almost embarrassed at the end, but they got the job done to get to a cup final.
"I don't like what I see from this United team. They are hard to like. They play in moments. We go on about leadership and character, but I don't see any of this from this United group.
"You do not give a Championship team hope they can get back into the game from 3-0 down, but that is what this United team does. They give up chances and goals, we've seen it all season, so I don't know why we're surprised. My goodness, they rode their luck at the end."
Robins: We played like Man Utd - and would have won if Wright had cut his toenail!
Coventry boss Mark Robins:
"If [the VAR offside decision is] the right call, it's the right call.
"They are blistering on the counter-attack so you have to be careful how soon you open up. We had to be careful. But we didn't really participate as an attacking force in the first half. Second half we made a change with the set-up.
"3-0 down then we start to play a little bit. It's easier to play when you're 3-0 down. Ellis Simms' goal looked like it could be a consolation. The second one, it gets everyone's tails up, the supporters are involved and that was what we were after.
"They started to drop away. They were leggy. We tried to exploit that and ended up getting the penalty for 3-3 and that's where things started to get interesting! They hit the bar, we hit the bar. Then the offside. I think it was a toenail.
"There's no criticism with anybody who shows the courage to pick up the ball and have a go with a penalty in that cauldron. I'm really proud of everybody. The supporters were outstanding. This FA Cup run will be spoken bout in Coventry circles for a long, long time.
"We've gone toe-to-toe with a Man Utd side and almost won it and lost it on a shootout. Had he cut his toenail we wouldn't be talking about penalties.
"We performed like Man Utd for that last hour. That's what I'm proud of. They were really good. That was more of a swashbuckling performance - and that's how Man Utd usually play."
Ratcliffe shows the strain as United almost blow it
Sir Jim Ratcliffe has taken over football operations at United but may wonder what he's let himself in for! Ratcliffe ran the London Marathon earlier on Sunday but looked exhausted watching on as Coventry threatened to pull off the greatest FA Cup comeback in history.
250/1? Coventry's incredible comeback in pictures
Coventry were priced up at 250/1 to draw when it was 3-0 by Sky Bet...
What's next?
Coventry switch their attention back to securing a play-off place in the Sky Bet Championship when hosting Hull City on Wednesday 24 April, kick-off 7.45pm.
Manchester United host Sheffield United in the Premier League on Wednesday 27 April, kick-off 8pm.