City's 11-game winning streak came to an end as they went down 1-0 at Sheffield Wednesday on manager Danny Wilson's first return to the club he played for and managed.
A Graeme Lee header from a long Leigh Bromby throw deep in stoppage time gave Wednesday the points as table-topping City crashed to defeat.
The last time City lost had been in an FA Cup replay when another former Wilson managed side, Barnsley, beat them 2-1 at Oakwell.
Back to back league victories for Wednesday had come courtesy of three penalties and on 11 minutes they thought they had won another when Jon Shaw appeared to be fouled in the box by Danny Coles, but referee Mark Cowburn waved play on.
City's neat football created an opening for Aaron Brown on 15 minutes but he fired over.
Wednesday had a second penalty shout turned down on the stroke of half-time when Coles looked to have handled under pressure from Guylain Ndumbu-Nsungu.
On-loan Adam Chambers spurned Wednesday's best chance on 53 minutes. The ball arrived at his feet after Shaw had made a nuisance of himself on the left, but from 12 yards out Chambers dallied and shot into the gleeful Bristol supporters.
Kevin Pressman, who had answered Chris Turner's SOS to return from his loan at West Brom, needed all his years of experience to tip away Tommy Doherty's goal bound drive on 56 minutes.
City were unlucky not to be in front just after the hour when Lee Peacock had a header deflected onto the underside of the crossbar by Steven Haslam.
Substitute Christian Roberts stunned Pressman's hands with 14 minutes remaining as City pressed for a winner.
Pressman had to be alert again on 81 minutes to divert another Peacock shot over.
At the other end, Shaw ghosted into the area, but he headed Haslam's cross wide.
But Lee made it an unhappy return for Wilson with his stoppage time header.