“ Judicial wisdom counsels against anticipating hypothetical situations in which the discretion of the trial judge may be abused and so call for the safeguard of the Fifth Amendment-cases in which the defendant would be harassed by successive, oppressive prosecutions, or in which a judge exercises his authority to help the prosecution, at a trial in which its case is going badly, by affording it another, more favorable opportunity to convict the accused. ”

“ Where, for reasons deemed compelling by the trial judge, who is best situated intelligently to make such a decision, the ends of substantial justice cannot be attained without discontinuing the trial, a mistrial may be declared without the defendant's consent and even over his objection, and he may be retried consistently with the Fifth Amendment. ”

“ We would not thus make them unduly hesitant conscientiously to exercise their most sensitive judgment according to their own lights in the immediate exigencies of trial for the more effective protection of the criminal accused. ”