Barclay retaliated by hitting the mirror, "expecting it just to ping back" and not to break.
He apologised to the car occupants and offered his phone number.
Ms Sullivan said the car followed Barclay and then the occupants got out and "beat him up".
Judge Peter Hobbs asked police to confirm this, but police prosecutor said it "was the first I've heard of it".
Judge Hobbs said that if police could corroborate the circumstances, he would discharge Barclay without conviction. But they could not, so Barclay was convicted and ordered to pay $259 in costs.
Barclay was in court with a sling around his arm. Ms Sullivan said outside the courtroom that he had had a shoulder injury that was aggravated when he was assaulted.