With the sun shining on the scene of his footballing farewell, the Scot saw his side throw away 3-0 and 5-2 leads as history was made in the Black Country. It was the club's first 5-5 draw since 1895.
Shinji Kagawa, Alex Büttner, Robin van Persie and Javier Hernandez all scored, and Jonas Olsson put through his own net, but a hat-trick by substitute Romelu Lukaku helped the Baggies ensure Sir Alex's career ended on a slightly disappointing note.
The champions came out to a guard of honour while both sides applauded the manager onto the pitch amid a tumultuous atmosphere inside the ground as the stage was set for an occasion befitting the great man’s last-ever stint in the dug-out.
With the packed away end roaring the Reds on, some of the football on show was simply sensational. As the ball was worked out of defence superbly in the sixth minute, Büttner's pass into the right-hand channel looked overhit but Hernandez chased it down and supplied a pinpoint cross for Kagawa to head down and past former United keeper Ben Foster.
Three minutes later, Kagawa ignored a stumble from the nearby van Persie to feed Antonio Valencia out wide. The raiding wing-back smashed the ball across goal and Olsson poked it past Foster to double the lead - a similar own goal to the ones netted in two of the past three matches with Albion by Steven Reid and Gareth McAuley.