An elite group of police officers have the job of identifying each suspect captured on CCTV.
Known as super-recognisers, these policemen have the ability to match faces on camera to those stored in their photographic memory.
This exceptional ability has helped prosecute hundreds of people who took part in last summer's troubles.
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The eagle-eyed resident spotted the mistake on signs just feet apart in south London.
The victim was attacked after bumping into a man at the ticket barrier at Putney Station.
The victim, who is his 40s, was found near the edge of palace grounds.