What is the test for making a motion for a mistrial?

"A mistrial should be granted if the court is apprised of prejudice that it judges incurable by admonition or instruction. (People v. Woodberry (1970) 10 Cal.App.3d 695, 708.) Whether a particular incident is incurably prejudicial is by its nature a speculative matter, and the trial court is vested with considerable discretion in ruling on mistrial motions. (Illinois v. Somerville (1973) 410 U.S. 458, 461-462.) Accordingly, it would be a rare case in which the merits of a mistrial motion were so clear that counsel's failure to make the motion would amount to ineffective assistance. Nonetheless, defendant could conceivably prove incompetence if his counsel's omission was shown to be grounded in ignorance or misapplication of the law rather than tactical considerations

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