There was cause for concern for Sirius, who sold the division's top scorer Robbie Ure to Sevilla just hours after throwing away a 2-0 lead to draw at home to Brommapojkarna last time out, as their lead at the top was cut to 12 points.
Still, with such a healthy advantage and without defeat in 16 matches, they would have been confident of getting back to winning ways at Mjallby, who were dumped out of the UEFA Champions League qualifiers in midweek and were hovering just above the bottom three after going winless in eight in Allsvenskan.
The match in Hallevik looked to be going to form when Sirius went in front just six minutes in, when the division's new top scorer Isak Bjerkebo cut in from the left, and when his shot from the edge of the area was deflected, it dropped nicely for Otso Liimatta to divert in from eight yards and score his first goal since signing from Halmstad.
It was 2-0 midway through the first half when a dreadful header back from Ludvig Svanberg allowed Liimatta to nick in ahead of goalkeeper Robin Wallinder and roll the ball into the empty net. But by injurying himself in the act of scoring, Liimatta was able to play no further part.
But Sirius seemed to be doing just as well without him, extending their lead 10 minutes later, when Bjerkebo raced onto a loose ball and kept on running, right into the heart of the MAIF box, before coolly finishing past Wallinder.
Whatever Karl Marius Aksum said to his players at the break, it had the desired effect. As did his substitutions - three of which he made at the break and two new additions combined less than a minute after the restart to get Mjallby on the board. Max Nielsen squared to Olle Lindberg, whose low effort went in off the post.
It was game on again in the 54th minute, when Sirius made a mess playing out from the back, and an Aki Samuelsen interception was picked up by Jacob Bergstrom, who applied the finish for 3-2.
The same duo combined again 13 minutes from time as Mjallby completed the comeback - this time Samuelsen played a precise ball over the top for Bergstrom, who shrugged off his marker, opened up his body and slot his second of the game between the goalkeeper's legs.
And with three minutes of the 90 remaining, the home side did the unthinkable, and it was the original combination of Nielsen and Lindberg that did the damage. Nielsen used his pace down the right wing to get to the byline and dig out a cross right onto the head of Lindberg, whose header went straight through the gloves of Celic, to give Mjallby the most incredible victory.
It's a first league win for Mjallby since beating Degerfors on 9th May, and their first home league victory since April. That moves them up to 10th for the time being, while Sirius have been beaten in the league for the first time since 29th September.