For the record - I WAS a cop. Not a frigging lawyer!
And now, in Legal Advice Hour, let's hear from our Defective Detective himself, mister FMH.... yay, crowd goes wild...
Based on what you've said, no. A fine could not be imposed without prima facie evidence. If it was imposed, it could be appealed. Chequered history or not, where's the evidence of the offence? IF the cop issued a fine and you appealed it, they would need to have the witness make a statement. Even then, there is a technicality that the fine should not have been imposed without evidence. "Gees, officer, I wonder if that was the lady that stepped out between 2 cars while on her mobile phone and I had to take evasive action to not run her over? Funny that she'd make a complaint about me when she was lucky I didn't clean her up!"
if the dodgy witness said you were speeding through a school zone for example, a cop might find you and pull you over, and as a first step determine if you're under the influence and posing some sort of risk. Duty of care. If not, then give you a talking to that you were witnessed allegedly doing something damn stupid and that's where it ends.