Manchester United thought they had equalised against West Ham around the 60th minute, only for Casemiro's headed goal to be chalked off by VAR.
Kobbie Mainoo delivered a superb pass into the box, and the Brazilian midfielder rose highest to powerfully head the ball past the keeper.
Initial celebrations erupted, but Casemiro's own reaction – looking straight at the linesman and gesturing – suggested he suspected he was offside.
The referee quickly pointed to the pitchside monitor for a VAR check.
After a brief review, the goal was ruled out for offside, and the decision appeared clear on replay.
Casemiro was fractionally ahead of the last West Ham defender when Mainoo played the ball.
United remain 1-0 down at the London Stadium, with Michael Carrick's side pushing hard for an equaliser after Tomas Soucek's earlier strike exposed defensive errors.
The disallowed goal has frustrated fans, but the offside call looks correct based on the evidence shown.
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