A Melbourne shisha lounge manager's conviction for raping a 16-year-old girl has been quashed and a retrial ordered after a legal bungle resulted in a judge spotting a juror walking the streets during verdict deliberations.

Historically, jurors were kept away from the outside world while they decided on their verdict following a trial – even kept together in hotels for days on end.

This statute has been relaxed, allowing people to go home at the end of each day and outside during court hours, but only if they were accompanied by a “jury keeper”.

It was a surprise then for the County Court trial judge Sandra Davis to spot an unaccompanied juror pass her while she was walking in the city on her lunch break just after the jury had retired to begin deliberating their verdict at the conclusion of the trial in March last year.

Judge Davis subsequently discovered not one, but 10 jurors had left the jury room - though one just ducked into the room next door to use the vending machine - in what she described as a “serious mistake”.