Russell Slade criticised referee David Coote after his error gifted Birmingham victory in a narrow win over Cardiff.
The Cardiff boss was furious with the official after he wrongly awarded a penalty for handball against Matt Connolly in the Bluebirds' 1-0 defeat.
The defender blocked James Vaughan's shot with his face but Coote pointed to the spot and Paul Caddis scored a first-half winner at St Andrew's.
It ended Cardiff's four-match unbeaten run to deny them a chance to move into the Sky Bet Championship play-offs spots and Slade admitted he had to be stopped from properly confronting Coote in the tunnel at half-time.
Slade said: "I would have liked to have got extremely close.
"I did have words with him at half-time and told him he got it wrong. With technology we knew pretty quickly he made a huge mistake. I'm not quite sure why, there were no players obstructing his view, he had a fantastic view of the situation."
And Slade said Coote would have been grateful for the cooling-off period managers have to wait before speaking to referees after a game.
"I think it's very much in his interest," he said. "The rules are I have to wait a little bit before I go in - a cooling off period they call that - but I did have words with him at half-time."
I did have words with him at half-time and told him he got it wrong.
The penalty was Birmingham's first home goal in 432 minutes and lifted Blues above Cardiff into seventh, a point behind the top six.
Boss Gary Rowett said: "At the time I couldn't see (the penalty), the lad had his arm up so it was very difficult to tell whether it hit his arm.
"I saw it afterwards and it's hit him clear in the face, the only thing the referee could potentially see was his arm is up next to his head so he maybe thinks there's contact. You have to be 100 per cent sure in that situation and he can't have been."