Judge Anthony Morris QC had attended a ceremony to mark his departure from the court prior to Ridwaan Shaikh's hearing.
Shaikh, 22, was cleared of murdering 21-year-old Bradley Cresswell but found guilty of manslaughter.
He had to wait for the verdict after the trial judge was "unavoidably delayed", court staff said.
The court heard that Shaikh, of Ilford, London, punched Mr Cresswell after they had been to Hydeout Nightclub, Watford, on 17 September.
Mr Cresswell, of South Oxhey, never regained consciousness.
Shaikh will be sentenced at the same court on Thursday.
Prosecutor Benn Maguire said the defendant attacked Mr Cresswell in the street "without warning" in revenge for an earlier incident in the club.
Following his arrest, Shaikh told police he acted in self-defence and hit Mr Cresswell with an open hand.
He denied both murder and manslaughter.
The jury deliberated for nearly 14 hours before clearing Shaikh of murder but convicting him of manslaughter.
The defendant wept in the dock while a member of the Cresswell family shouted "yes" as the verdict was read.
A defendant had to wait 45 minutes to learn his fate after a judge got stuck in a lift at the Old Bailey.