The notoriously hard-to-please panellist says he aimed to give a seven -- a generous score for him -- but ended up giving a four to one poor dancer.
"Of course, because it was a low score I then had to find as many things wrong with it as I could," he said.
The choreographer, whose tenth series of Strictly finished last month, did not name the hapless contestant.
"It was a four but I wanted to give a seven you see, so it was a bit of a disaster."
He added: "It's live television -- what can you do, darling?"
Revel Horwood admitted the gaffe during an edition of BBC2's Sarah Millican Television Programme.
Millican also dresses as a naughty nurse as she teams up with guests from BBC1 series, Casualty.
The comedian is joined by Tony Marshall, Suzanne Packer, Charles Dale and Azuka Oforka from the show.
She nipped off to get changed into a PVC outfit for a scene after stepping behind a curtain, telling her guests: "I've got my own uniform -- it's wipe-clean."