Hypothetically;
If I sit on a jury deciding a persons guilt or innocense possessing say , a couple of joints, and I think it's a frivolous charge based on (I guess, I don't know), the law that says a joint is illegal possesion, I can, while deciding, voice my opinion that this particular law is stupid and 'we' could find the defendent not guilty on the fact that 'we' in this case determine the law illegal?
No, you keep your mouth shut and steadfastly state during the jury deliberations that "The Prosecution has not proven the defendant's guilt beyond a reasonable doubt".