Birmingham slipped to their first defeat under new manager Steve Cotterill after losing 2-0 to Millwall at The Den.
Left-back Maxime Colin poked the ball into his own net just 63 seconds after the break following Steve Morison's dangerous low cross into the six-yard box.
Summer signing Ryan Tunnicliffe sealed the victory 14 minutes from time with his first goal for the south London club.
This victory was the Lions' first at home over their opponents in 10 attempts and also marked their fourth win from their last five Sky Bet Championship games at The Den.
The hosts had earlier dominated an opening 45 minutes which were low on action, with Morison, Lee Gregory and Aiden O'Brien all spurning half-chances.
Morison headed wide from George Saville's free-kick before strike partner Gregory missed the target with a powerful drive on the turn. O'Brien flicked narrowly over following Jake Cooper's far-post knockdown after 17 minutes.
On his 150th appearance for the Blues, Jonathan Grounds had a 25-yard free-kick deflected en route to goal after 42 minutes.
Striker Isaac Vassell headed straight at Millwall goalkeeper Jordan Archer from Jacques Maghoma's subsequent cross, before missing an even greater chance seconds before the break.
Having latched on to David Davis' through-ball, the former Luton forward fired inches wide of the post from an acute angle after fooling Cooper with a neat dummy.
The deadlock was quickly broken after the break, as Colin - under pressure from O'Brien at the back post - diverted the ball past Tomasz Kuszczak and into his own net from Morison's delivery.
On his return from injury, Morison forced Kuszczak into an excellent save 17 minutes from time with a powerful right-footed volley.
The hosts sealed their victory soon after, as Tunnicliffe slotted home from 15 yards after substitute Tom Elliott's stinging, curling drive was parried by Kuszczak only as far as the onrushing former Fulham midfielder.
Jed Wallace almost made it three 180 seconds later as his cross-turned-shot looped onto the roof of the net.
City rarely called Archer into action after the break, although he made a smart reaction save in the 64th minute to deny Che Adams from a tight angle.
Maikel Kieftenbeld saw his 30-yard strike deflected into the Millwall goalkeeper's arms by the energetic Saville and substitute Lukas Jutkiewicz headed over from five yards in stoppage time.
Defeat leaves Cotterill's side one place above the relegation zone, a point above Burton, while Millwall leapfrog Sheffield Wednesday and QPR into 15th.