The bright sunshine at Spotland disappeared ten minutes from the end and with the rain came all the drama.
There was a disturbance in the Scunthorpe side of the ground and while stewards and police were dealing with that, Rochdale were getting a penalty which clinched the game.
The referee ruled that Cliff Byrne had been guilty of shirt-pulling on Mikael Antoine-Curier and pointed to the spot. Leo Bertos, who had scored five minutes earlier with a right-foot shot from the edge of the penalty area, took the spot kick which was well saved by goalkeeper Sam Russell.
As Scunthorpe celebrated, the referee ordered a retake ruling that Russell had left his line and this time Ged Brannan took over from Bertos and duly netted.
Earlier, Richard Kell of Scunthorpe had been given a red card for a 37th minute foul on Rochdale's Sean McClare.
But with ten men Scunthorpe battled hard and with better shooting they could have had an unexpected lead.
The chances were there but the takers were not. The match could have gone either way with Scunthorpe doing extremely well despite their deficiency in numbers. Steve MacLean twice went close for United and Steve Torpey had one or two strokes of bad luck when he went looking for a winner.
But as the rain started to replace the bright sunshine in the last ten minutes it all went wrong for United. That stubborn defence began to crack as Rochdale piled on the pressure to remove the indignity of not beating ten men.
Bertos got what looked like being a winner on 82 minutes and in the dying seconds Brannan made sure. But United were not dead yet and all but got the ball home in a late goalmouth scramble.
Scunthorpe manager Brian Laws said after the match: "I think that Richard Kell deserved to be sent off and he will be fined the maximum. But I cannot agree about that penalty.
"It was a magnificent save by Sam Russell and he never left his line when the penalty was being taken."