Not for the first time, three letters dominated this weekend's Premier League action: VAR.
The referees' governing body described it as a "significant human error" - but Liverpool say that explanation is "unacceptable", claiming "sporting integrity has been undermined".
BBC Sport looks at the key questions.
What happened?
The game was goalless when Luis Diaz's 34th-minute strike was ruled out on the field for offside.
The decision was incorrectly upheld by VAR, with play cleared to continue despite still images showing Tottenham defender Cristian Romero playing Diaz onside.
It is understood the correct procedure was followed for the offside decision but the mistake was down to human error.