The federal judge presiding over the Roger Clemens perjury case has declared a mistrial. Judge Reggie Walton made his decision after prosecutors exposed the jurors to evidence he ruled inadmissible. The government lawyers’ blunder was a mistake Judge Walton said even “first-year law students” would have known to avoid. Michael McCann,  writes the “Sports and the Law”  column for Sports Illustrated and director of the Sports Law Institute at Vermont Law School, has been following this story.