I read once about a case where the foreman on the jury (in a personal injury suit) was both a lawyer and a state senator. The judgement was set aside on the basis that he exercised undue influence by explaining the law to other jurors (thus usurping the judge's role).

Of course the law is full of funny cases (like the time the Supreme Court filed a per curiam, and then along with it a unanimous partial concurrence). Figuring out what was meant there is... well, something lawyers can argue about forever.