Violence erupted at a Broughty Ferry hotel just minutes into the new year.
Dale Stephen admitted attacking Matthew McCardle at the Fort Hotel on Fort Street on January 1 last year.
Dundee Sheriff Court heard the pair had bumped into each other while walking across the bar area.
Mr McCardle pushed Stephen, who responded by headbutting him.
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Fiscal depute Gavin Burton said: “The incident took place at the Fort Hotel bar at around 12.50am on a very crowded dancefloor.
“As the complainer was passing by, the accused and the complainer bumped into each other.
“The complainer pushed the accused away and the accused reacted by stepping forward and butting the complainer to the face.”
The incident was captured on CCTV and Mr McCardle was said to have suffered a bloodied nose and swelling.
Stephen, of Belltree Gardens, pleaded guilty to carrying out the assault on the day he was due to stand trial.
Defence solicitor Ian Myles said the 25-year-old wanted to apologise to the court and to Mr McCardle.
Mr Myles told Sheriff Lorna Drummond that Stephen had hoped to resolve the case at an earlier stage had the Crown accepted that Stephen was reacting to being pushed.
He said: “It was over in an instant. He has fully co-operated with police officers and accepted his culpability.”
Stephen was fined £720.