Victory over Halmstad last time out had taken Sirius top of the pile after Hammarby had briefly held first place, and after Bajen beat Elfsborg two days prior, Sirius again only needed a point against a struggling side to return to the summit.
But Kalmar had other ideas, and are now six points clear of the drop zone with as many still to play for thanks to a narrow home win. The goal came with the half-hour mark approaching, when Vilmer Tyren drove at the away defence before firing, and though David Celic kept the effort out, he could only palm it straight to Amara Camara, who turned in the rebound.
Victor Svensson failed to convert the Uppsala side's best chance of the first half, and despite 70% possession after the break and 20 shots in total, Sirius were unable to find an equaliser, forcing Samuel Brolin into just two second-half saves on what could be the defining afternoon in the title race.
It's one win and three defeats in four for Sirius, whose +23 goal difference is 14 goals worse than Hammarby's. Now two matches unbeaten, Kalmar can guarantee survival by making that three in a row against Orgryte next time out.
Now six points behind the top two, albeit with a goal difference better than Sirius', Djurgarden are all but out of the title race after they were beaten by Halmstad, who stay bottom but are now level on points with both Degerfors and Orgryte.
The visitors conceded a similar goal to the one Sirius let in - midway through the first half, Ludvig Arvidsson's strike from range was kept out by Jacob Rinne, but following up to bundle in the rebound was Omar Faraj.
Ten minutes after the break, a long ball forward caused havoc in the Halmstad back line before breaking to Jeppe Okkels, who guided the equaliser into the bottom corner.
Djurgarden looked the more likely to go on an score the winner, only to be caught on the break three minutes from time, as Hussein Carneil sent Marvin Illary on his way - the substitute still had plenty to do, but he beat the last defender before slotting the ball past the goalkeeper.
It's two wins in four now for Halmstad, who have the relegation play-off place in their sights, while just one win in four has curtailed Djurgarden's hopes of a first title since 2019.
That result gave Hacken the chance to draw level with Djurgarden in the race for Europe, but it's a chance they didn't take as the Stockholm side's rivals AIK did them a favour by earning a first win in three and ending Hacken's eight-match unbeaten run in the league.
AIK were ahead at the break after Sixten Gustafsson chested the ball up and produced a bicycle kick, which would have gone just wide had Johan Hove not stretched out a leg to turn it home.
The visitors were back level just four minutes into the second half after Gustav Lindgren played the ball through for Mikkel Rygaard to slot home, but just two minutes later Gnaget were back in front as Axel Kouame pulled the ball back for Charlie Pavey to drill past the goalkeeper on his first appearance since May.
This time, the lead lasted just 10 minutes - Adam Lundkvist's pass from the left found Adrian Svanback at the edge of the area, and he unleashed a tremendous first-time hit that flew into the top corner.
Kristoffer Nordfeldt stood no chance with that shot, but in the 90th minute, he caused carnage of his own when his long ball forward evaded three Hacken defenders, allowing Eskil Edh a free run at goal, which he took to score his first of the season and hand AIK their third lead of the match.
This time they held on to move up to eighth place with a first win in three, while Hacken's loss meant all four teams now sitting second to fifth all were beaten in this round.
It has taken until round 28 of 30, but champions Mjallby were finally confirmed as safe thanks to Degerfors and Orgryte failing to win the day before they picked up their second goalless draw in a row against Brommapojkarna, who have now gone three without scoring.
The Bromma Boys still dominated proceedings with seven shots on goal to the visitors' one, but it was an afternoon to forget for Oliver Zanden, who missed three big chances for the hosts, whose point ensures they can finish no lower than 11th, a place above MAIF.
There were no goals either in Gothenburg, where Goteborg's season continued to fizzle out - they are 10th having drawn four and lost one of their last five, while Malmo have won just one of their last five and are at risk of finishing lower than their sixth-place finish a year ago.
Tobias Heintz struck the post for the home side, while Sead Haksabanovic and Otto Rosengren both went close for the visitors in a match where both teams will feel they did enough to win it
Follow the Allsvenskan title race and relegation battle on Flashscore.