Manchester United threw away a two-goal lead in the final seven minutes as Everton blew the Barclays Premier League title race wide open in an incredible game at Old Trafford.
Instead of setting the scene for an ultimate revenge act at the Etihad Stadium on April 30, United will meet Manchester City knowing they cannot afford to lose if a calamity even greater than the one that unfolded here is not to take place.
Behind to Nikica Jelavic's header, United twice established winning positions as Danny Welbeck and Nani sandwiched a Wayne Rooney double.
But Everton refused to surrender to what seemed like the inevitable defeat.
Marouane Fellaini had already offered a warning before Rooney struck the 180th goal of his United career and 33rd of the season, just one short of his highest haul.
But when Jelavic grabbed another, the hosts allowed Steven Pienaar far too much space inside the United box a couple of minutes later and the midfielder duly buried the goal that could provide the turning point in a season filled with so many amazing results.
Though it had been almost 20 years since Everton's last success on this ground, this was always viewed as a key game
With 10 points from their last 12, the Toffeemen represented a formidable obstacle and the return of Pienaar merely offered more reason for optimism amongst the Manchester City faithful.
And the Merseysiders certainly did City proud, starting on the front foot, defending stoutly and thoroughly deserving their lead when it eventually came.
Jelavic had already sprung the United defence once, early on, when Pienaar sent him through, but David de Gea was equal to the shot.
Leon Osman had a couple of opportunities, Sylvain Distin guided a header wide and United old boy Darron Gibson chanced his arm with a trademark thunderbolt, which flashed wide.
However, United had no answer when Tony Hibbert crossed deep from the right touchline.
Jelavic won the aerial duel with Rafael at the far post, lifting his header over De Gea and just inside the goal.
It was the £5.5million man's seventh Everton goal and sixth in six games, emphasising his status as one of the bargain buys of the season.
A wave of anxiety swept around Old Trafford as United fans feared another sizeable chunk was in danger of being carved off what, just a fortnight ago, seemed an impregnable lead.
After a couple of decidedly dodgy penalty awards involving Ashley Young over the past fortnight, it was perhaps no surprise referee Mike Jones was unimpressed when Rafael went down under Osman's challenge, although it did appear to be a foul.
United needed to come up with something fast.
And, restored to the starting line-up after a nine-match absence in place of Young, whom Ferguson insisted had not been dropped for disciplinary reasons, Nani provided it.
The Portugal winger curled over a brilliant cross from the left which dropped just over Phil Neville and perfectly for Rooney to head in.
In a sense though, an equaliser was neither here nor there. The difference between a two or three point lead largely irrelevant considering City had a better goal difference.
United needed three points. And, in the space of two minutes just before the hour, it seemed they had been secured.
Everton complained that Osman was down and in pain when the hosts went in front.
However, they had many chances to nullify any potential danger before Nani showed admirable desire to out-jump Neville on the edge of the area and nod down for Welbeck.
After side-stepping John Heitinga's rash challenge, the youngster calmly curled a superb shot into the top corner.
Old Trafford had not stopped celebrating when Welbeck fed Nani, whose chip sailed over Howard and bounced into the empty net.
It should have been all over and still seemed that way despite Fellaini smashing home a brilliant volley and Rooney had soon struck again to restore their two-goal cushion.
In truth, the drama was only just starting.
With seven minutes left Jelavic capitalised on unusual hesitancy inside the United penalty area and drove a precise shot into the bottom corner.
Even then, what happened afterwards came with jaw-dropping surprise, Fellaini creating space for himself inside the penalty area before sliding a pass through to an unmarked Pienaar, who planted a first-time shot beyond De Gea.
The bedlam of earlier had been replaced by a different kind of mayhem as United tried to get a grip on the calamity that had just unfolded.
Phil Jones and Javier Hernandez were thrown on in a desperate attempt to salvage the situation.
But even in five heart-stopping minutes of injury time there was to be no final twist as Tim Howard tipped over Rio Ferdinand's goalbound effort.