Young striker Bukayo Saka fell in a duel on his way to a free position - and Arsenal thought referee Mike Dean would blow.
So did Dean. For free kick for Sheffield United and then he gave Saka a warning for filming.
Something that became too much for Fredrik Ljungberg, Arsenal's assistant coach. And Dean picked up the yellow card again.
Sheffield United won by 1-0. Lys Mousset from close range bumped into the victory goal after a corner in the first half.
Arsenal created some dangerous chances in the first half, but weighed in easily against hard-working, physically Sheffield. After nine rounds played, Arsenal are fifth and Sheffield U nine in the table.