Aviva Premiership leaders Saracens let slip an eight-point lead as they lost 23-17 to Gloucester at Kingsholm.
The visitors dominated the first half and scored the game's first try in the 53rd minute through Schalk Brits but Ollie Thorley and Tom Hudson crossed in the final quarter to secure victory for Gloucester, who move up to third in the table.
Billy Twelvetrees converted both tries and kicked three penalties while Alex Lozowski kicked nine points and Ben Spencer landed a late penalty for Saracens to claim a losing bonus point.
The leaders were able to include George Kruis and Lozowski after both were released from the England squad, but number eight Jackson Wray was a late withdrawal with a neck injury.
Twelvetrees put Gloucester in front with a penalty after eight minutes but this was soon nullified by one from Lozowski.
The opening 15-minute period was disjointed as the referee, former Harlequins scrum-half Karl Dickson, marked his Premiership debut by awarding a succession of penalties.
Lozowski could have given his side an advantage but was off-target with a 30-metre drop goal attempt, but moments later his simple penalty gave Saracens a 6-3 lead after 18 minutes.
An attempted clearance from Billy Burns was charged down by Lozowski to allow the visitors a period of pressure but a knock-on from Schalk Burger relieved the danger.
But Saracens were in control. Strong runs from Sean Maitland and Brits tested the home defence before Lozowski missed a penalty as the visitors continued to batter their opponents' line.
Resolute defending from Gloucester kept the visitors out but a minute before the interval Lozowski knocked over his third kick for a deserved 9-3 half-time lead.
After the restart, Gloucester raised their game to compete with the intensity levels of the visitors and were rewarded with a second penalty from Twelvetrees.
The hosts then took off skipper Will Heinz in favour of the more attacking youngster Ben Vellacott but it did not pay immediate dividends as a burst from Burger set up the first try of the game for Brits.
Twelvetrees kept his side in contention with a third penalty to pull the score back to 14-9 going into the final 20 minutes.
A clever inter-play from Vellacott and Andy Symons took the home side into Saracens' 22 before a well-judged kick from Burns saw Thorley collect and score.
An excellent touchline conversion from Twelvetrees put his team ahead for the first time. And they completed a remarkable turnaround when Vellacott send Hudson in under the posts, before Spencer's last-minute penalty.