A 22-year-old man - reportedly a member of the victim's family - is accused of lunging at the teen as he entered Brisbane Supreme Court on Monday.
The 22-year-old allegedly stabbed the 18-year-old defendant with a pen in the back before he was restrained by security.
The courtroom was cleared as multiple police arrived.
An elderly man who was reportedly a supporter of the victim's family collapsed outside the courtroom and was taken by paramedics.
"Initial investigations indicate two males were involved in a physical altercation, with one sustaining a non-life-threatening wound from a pen," a police statement read.
"The 18-year-old man sustained a minor injury to his back."
The 22-year-old was on Monday charged with assault occasioning bodily harm whilst armed and making threats.
He is due to appear in court on April 10.
The 18-year-old's sentencing for murder was adjourned to a later date.
He was to be sentenced on Monday over the fatal stabbing of a 19-year-old in May 2024.
Police had earlier alleged two men were seen fighting on the footpath in the Brisbane suburb of Acacia Ridge before the 19-year-old was stabbed multiple times.
The victim was taken to hospital with leg and stomach wounds but died the next month.
Police located a hunting knife up to 30cm long nearby.
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