West Brom defender Nat Phillips was at fault for the second successive derby as Birmingham won 1-0 at The Hawthorns.
Phillips, whose back pass let in Fer Lopez in the 1-0 defeat to Wolves last time out, gave away a 41st-minute penalty scored by August Priske, who he fouled.
Albion lost their 10-match unbeaten home league run, the first since James Morrison took charge in February.
In a lively first 20 minutes, a mis-hit lobbed back pass by Phil Neumann caused panic for Birmingham.
Goalkeeper James Beadle dashed well outside his box to beat Aune Heggebo to the loose ball and clear.
At the other end, Albion goalkeeper Max O'Leary made a superb reflex save to deny Jay Stansfield's sliding shot from point-blank range.
Albion broke at speed and Heggebo steered a shot against the foot of a post with Beadle beaten.
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Jhon Solis got his footing wrong after Heggebo's poor clearing header from Alex Cochrane's cross, the Colombian midfielder shinning his effort from 12 yards.
Felix Horn Myhre should have put Albion ahead in the 28th minute, heading just over the crossbar with more than half of the goal to aim at.
Albion went close again when Callum Styles curled a low free-kick around the three-man wall and Beadle tipped the ball behind.
Blues took the lead from the spot after Phillips caught Priske in the air with a clumsy challenge from Neumann's deep cross.
Priske coolly side-footed the penalty high to O'Leary's left for his second Sky Championship goal of the season.
Albion had O'Leary to thank for preventing a second goal with a superb reaction save to divert Neumann's fierce downward header over the bar just before half-time.
No fewer than four Albion players slid in to block Solis' shot from six yards after O'Leary got a hand to Cochrane's corner.
Blues substitute Patrick Roberts appealed for a penalty after falling following minimal contact from Chris Mepham, but the referee was unmoved.
Ousmane Diakite was not far away with a long-range angled drive as Albion tried to find an equaliser.
Cutting in from the left, centre-back Mepham was the unlikely figure dancing through Stansfield and Solis before forcing Beadle to smother.
Substitute Barney Stewart's glancing header was tipped over by the keeper from Jimmy-Jay Morgan's cross.
Stansfield curled straight at O'Leary after substitute Max Bird won the ball high up for Blues.
West Brom right-back Brayann Pereira sliced high and wide after a 40-yard run from inside his own half.
At the death, O'Leary's foot denied Birmingham substitute Demarai Gray at the near post.
West Brom's James Morrison:
Birmingham's Chris Davies:
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