SUPER-SUB Romelu Lukaku bagged a sensational hat-trick as Albion denied Sir Alex Ferguson the win he so desperately craved on his 1,500th and final game in charge of the champions by netting three times in the last ten minutes.
United surged 3-0 up inside 30 minutes thanks to Shinji Kagawa’s opener, a Jonas Olsson own goal and an Alexander Buttner strike.
Goals either side of the interval from James Morrison and Lukaku reduced the arrears to 2-3 but further efforts from Robin van Persie and Javier Hernandez restored United’s three-goal cushion.
But Lukaku got the Baggies back in it with his second on 81 before Youssouf Mulumbu added a fourth in the same minute.
And the roof came off four minutes from time when Lukaku bundled home his third of the afternoon - and 17th of the season.
What an afternoon!
Steve Clarke made four changes for the final game of his first season at the helm, with Billy Jones, Liam Ridgewell, skipper Chris Brunt and Graham Dorrans coming in for the injured Steven Reid and benched trio Goran Popov, Markus Rosenberg and Lukaku.
Ferguson made six changes. In came Anders Lindegaard, Antonio Valencia, Jonny Evans, Alexander Buttner, Anderson and Tom Cleverley for David de Gea, Patrice Evra, Rio Ferdinand, Nemanja Vidic, Paul Scholes and the injured Danny Welbeck.
The ground rose as one to salute Ferguson as he made his way through a guard of honour – formed by coaching staff and players from both sides – just before kick-off.
And the United boss had plenty to cheer about during the opening nine minutes as United surged 2-0 up.
After an uneventful start, United’s opener came like a bolt out of the blue. Javier Hernandez chased down a hopeful ball by Cleverley before picking out Kagawa, who had the freedom of the Baggies’ six-yard box to head past the helpless Ben Foster.
Three minutes later, there was another body blow for the Baggies. Valencia ran at Ridgewell before whipping in a low cross which the luckless Olsson turned into his own net.
It was an opening befitting the champions of England and Brunt had to produce a vital block to prevent Phil Jones inflicting further damage on 14 after a left-wing corner had wreaked havoc inside the home side’s penalty area.
Albion understandably looked shell-shocked but desperately tried to gain a foothold in the game. They mustered their first attempt at goal on 21 but Brunt’s ferocious 25-yard drive was blocked in its infancy by Anderson.
The hosts had another big scare on 24 when Michael Carrick picked out Hernandez unmarked at the back post but the Mexican wastefully headed well wide.
However, it WAS 3-0 on the half-hour mark. Cleverly slipped in Buttner who rifled an unstoppable effort into the far bottom corner of the net with his left foot.
Albion produced their best moment of the game thus far two minutes later when Long outmuscled both Buttner and Evans to header Brunt’s deflected centre over.
However, on 38, United almost added yet another goal. Valencia hooked the ball into the box after chasing down a lost cause and Hernandez poked just wide from seven yards.
But the Baggies earned themselves a lifeline two minutes later when they reduced the arrears. After Albion won possession deep in the United half, Youssouf Mulumbu fed Dorrans whose low cross was swept home by Morrison from eight yards.
There was still time for two great chances at both ends.
On 43, Valencia combined with Cleverley to tee up Hernandez whose 15-yard shot was blocked by Foster. However, the keeper had to make an even better save as the rebound bounced goalwards off McAuley, showing terrific reflexes to palm the ball over the bar.
In the first minute of stoppage time, Albion almost made it 3-2. Morrison pounced on a Jones mistake and played the ball across goal to Long, whose first-time effort was diverted wide by Evans’ brilliant last-ditch tackle.
But it really was game on five minutes after the break. Dorrans and Morrison worked the ball to sub Lukaku, who had replaced Ridgewell during the interval, and his low 20-yard effort was too good for Lindegaard, who could pushed the ball in off the far post.
United regrouped and Anderson directed Valencia’s pull-back straight at Foster from 20 yards.
However, on 53, the visitors restored their two-goal cushion. Valencia surged to the by-line before cutting the ball across goal and van Persie nipped in front of Olsson to fire home.
Back came Albion and Mulumbu was unlucky to see a foraging run inside the box come to naught before Dorrans fired wide from 25 yards.
But United looked like scoring every time they piled forward – and they added a fifth on 63. Sub Ryan Giggs, who been on the field barely three minutes, whipped over a terrific left-footed centre which Hernandez tapped home from six yards.
Yet Albion should have been back on the scoresheet on 67 in this most bizarre of encounters. Olsson was completely unmarked as Dorrans’ corner dropped to him in the six-yard box – but he volleyed over the gaping net.
Moments later, Dorrans was off target with an ambitious effort from the edge of the box.
But Albion came pressing and Lukaku was only a whisker away with a low 30-yard free-kick which clipped the United and spun inches wide.
The on-loan Chelsea man went even closer moments later when he placed the ball agonisingly wide from 15 yards after being picked out by Morrison.
The action continued to come at both ends. Buttner inadvertently clipped the Albion bar with a wayward cross before Dorrans fired over from Lukaku’s pull-back.
Then, on 78, Giggs saw an audacious chip land on the roof of the Albion net.
Albion sensationally made it 3-5 nine minutes from time when he raced onto sub Rosenberg’s perfect pass and slipped the ball past Lindegaard for his 16th goal of the season.
And there was bedlam in the home sections in the same minute when Mulumbu added a fourth, calmly sliding home Jones’ low cross.
As if that wasn’t unbelievable enough, it was 5-5 four minutes from time. Dorrans’ cross was forced across goal by Mulumbu at the far post and Lukaku outmuscled sub Ferdinand to bundle the ball past Lindegaard and bag his hat-trick.
ALBION (4-2-3-1): Foster; Jones, McAuley, Olsson, Ridgewell (Lukaku HT); Yacob, Mulumbu; Dorrans, Morrison (Fortune 79), Brunt; Long (Rosenberg 79). Subs not used: Myhill (gk), Dawson, Tamas, Popov.
UNITED (4-4-2): Lindergaard; Valencia, Evans (Ferdinand 83), Jones, Buttner; Anderson, Carrick, Cleverley (Giggs 60), Kagawa (Scholes 69); van Persie, Hernandez. Subs not used: De Gea (gk), Evra, Vidic, Januzaj.
GOALS: ALBION – Morrison (40, Lukaku (50, 81 & 86), Mulumbu (81). UNITED – Kagawa (6), Olsson (own goal 9), Buttner (28), van Persie (53), Hernandez (63).
BOOKINGS: UNITED – Scholes (foul 82).
REFEREE: M Oliver (Northumberland).
ALBION STARMAN: Romelu Lukaku.