Arsenal missed out on the chance to move up to third place in the Premier League with a 1-0 defeat against Sheffield United.
Unai Emery's men entered the match unbeaten in eight matches, including five in the top flight, but came unstuck at Bramall Lane.
The only goal of the game was scored by Lys Mousset half an hour in, coming shortly after Nicolas Pepe had failed to round off a swift counter.
Arsenal had a strong penalty appeal turned down and ultimately could not find a way through, the result leaving the Gunners fifth in the table and winless in four league visits to this ground.
The visitor was punished for Pepe's close-range miss by Mousset's opener eight minutes later.
Jack O'Connell headed down a corner into the six-yard box, where Mousset was waiting to pounce to double his tally for the season.
Arsenal was further frustrated when Bukayo Saka was cautioned for simulation when he went down in the box, despite arguing that contact was made by Enda Stevens.
Granit Xhaka had a long-range drive pushed aside by Dean Henderson and the same player glanced over just before half-time.
Pepe was also denied by Henderson from a free-kick and Sead Kolasinac headed over from a good position, but Arsenal was unable to find an equaliser on another day of frustration for Emery.