Sir Alex Ferguson was denied victory in his 1,500th and final game in charge of Manchester United as Romelu Lukaku's hat-trick earned West Brom a 5-5 draw.
The champions went 3-0 up through Shinji Kagawa, Jonas Olsson's own goal and Alexander Buttner's fine finish.
James Morrison reduced the deficit and Baggies' substitute Lukaku made it 3-2, before Robin van Persie and Javier Hernandez put United 5-2 ahead.
But Lukaku's late double and Youssouf Mulumbu denied Ferguson a winning end.
Watching his side concede three goals in the space of five minutes late in the game was hardly the finish Ferguson wanted at the end of a magnificent managerial career.
But the retiring Scot still managed a smile and a wave before disappearing down the tunnel for the last time.
Despite the crazy finish at The Hawthorns, United ended the season 11 points clear of Premier League runners-up Manchester City, the most comfortable winning margin since Chelsea beat Arsenal by 12 points in 2004-05.
If Ferguson's official retirement party was at Old Trafford seven days earlier, this was the final goodbye.
Nearly 10,000 days since his first match in charge of United, an inauspicious 2-0 defeat against Oxford United, Ferguson took his place in the dugout for the final time after players from both sides had formed a guard of honour.
Dressed in blazer and tie, Ferguson acknowledged all four sides of the ground as a sell-out crowd rose in appreciation of the Scottish mastermind who had accumulated 13 league titles, five FA Cups, four League Cups, two European Cups and one Club World Cup during 26 years in charge at Old Trafford.
Ferguson then sat back and watched his much-changed side dismantle West Brom in the opening 30 minutes before the home side roared back to snatch a thrilling point which sees them finish a creditable eighth in the Premier League table.
Though the occasion revolved around the 71-year-old, there was a match to be won and records to be broken.
Lukaku's second-half efforts ultimately denied United a Premier League record 29th win of the season, which would also have seen Ferguson's men equal their best points haul of 91 in the competition.
West Brom's chances of raining on Ferguson's parade seemed slim in the sixth minute when Japan international Kagawa punished some poor defending to nod home Hernandez's cross from close range.
Three minutes later it was 2-0, Olsson steering Antonio Valencia's low cross from the right past Ben Foster.
Such was United's dominance in the first half that there were times when the match had the feel of a pre-season friendly.
Ferguson did not even bother getting out of his seat to celebrate Buttner's 30th-minute goal, a powerful finish from a tight angle by the Dutch full-back after neat work by Tom Cleverley and the impressive Hernandez.
The Baggies reduced the deficit before half-time through James Morrison after he had got ahead of Phil Jones to steer home a cross from the right.
The introduction of Lukaku, on loan from Chelsea, at the start of the second half ultimately proved to be the turning point.
The Belgian found the net with a quality low finish from 20 yards before Robin van Persie's 26th league goal of the campaign, from a cross by Valencia, made it 4-2.
It looked all over when the tireless Hernandez added a fifth by steering home a Ryan Giggs cross.
But after Lukaku converted a neat pass in the 81st minute to bring the hosts back within two goals, Mulumbu made it 5-4 moments later following a cross from the right.
The comeback was completed in the 86th minute when Lukaku bundled the ball over the line from close range at the second attempt after United's defence had failed to clear.
The final whistle was greeted with relief on the United bench, Ferguson just about managing a smile as he made his way from the dugout.
With one final wave, he vanished down the tunnel for the last time.
Truly the end of an era.
West Brom manager Steve Clarke:
"It was a great occasion. That game will be remembered for a long time.
"I am not sure Sir Alex enjoyed it that much, but he was quite complimentary about our season and I wish him all the best.
Clarke on hat-trick scorer Romelu Lukaku:
"Romelu is a Chelsea player and he will go back there. We wish him well. If Chelsea wish to let him out on loan next season then we would love to have him back."