Legal complexities

Somewhere I've read that Tennessee has a 'risk assumption' law, meaning if you choose to do something risky you can't sue over any consequences. Is this true? How would riskiness be decided? What onus is on vendors to declare the risk before taking your money - etc, etc.. Could be a new bonanza for lawyers; you can sue, or you can have a court case to decide if you [i][u]can[/u][/i] sue - and with any luck you'll be able to do both?

In a US context I've also read that a judge can bias the jury by instructing them which laws can be considered in the case - also that a jury can acquit despite clear evidence of guilt. I seem to recall seeing a movie based on this, long ago. I don't know if it was based on fact. If it's true would the accused be safe from being charged again with the same offence?