Although the defendant now urges that the judge could have ordered a mistrial had he felt the need to empanel more than twelve jurors, where jeopardy has attached, mistrial, and the possibility of second jeopardy, must be used cautiously. See Commonwealth v. Steward, 396 Mass. 76, 79 (1985); Commonwealth v. Phillips, 12 Mass. App. Ct. 486 (1981). The defendant not only failed to raise this double jeopardy claim below, despite the judge granting a recess at the prosecution's request so that the office of the district attorney could investigate whether as a matter of law empanelling the additional jurors was permissible, but also affirmatively approved of the procedure used.

See Commonwealth v. Steward, 396 Mass. 76, 79-80 (1985); Commonwealth v. Barton, supra at 519. Cf. Commonwealth v. Phillips, 12 Mass. App. Ct. 486, 487-488 (1981). The possibility was apparent; the defendant raised it, and it was not explored.