An English lady once asked the Lord Chief Justice what was necessary in order to win a case in court. He replied: "First, you need a good case, then you need good evidence, then you need good witnesses, then you need a good judge, then you need a good jury, and then you need good luck."
Wellman, The Art of Cross-Examination, Ch. 6, p. 143. As this quote suggests, there are a lot of things that can go wrong. Improper testimony or argument must be objected to, and if appropriate, a curative instruction should be requested. However, the curative instruction, if given, may be insufficient to remove the prejudice, and it may no longer be possible for your client to get a fair trial. When that happens, the defense counsel must move for a mistrial.
This, of course, is not to be done by blurting out in open court, in front of the jury, "I move for a mistrial." No, the way it is done is to rise and ask the court if you may approach. Then you must move for a mistrial at the bench, out of the hearing of the jury. The trial judge may not want to hear this, and may refuse permission to approach, but you must be insistent, and in any event you must make your motion at the earliest opportunity. The denial of a request for relief is preserved for appellate review if the record shows that it was presented to the trial court "at a time when the trial court could have corrected the error." McCallum v. State, 81 Md. App. 403, 419, 567 A.2d 967.
Failure to do so will likely result in waiver of any right to take the issue up on appeal. To preserve the argument that a curative instruction to disregard inadmissible evidence cannot cure the error, a motion for mistrial is necessary. See Medical Mutual v. Evans, 330 Md. 1 (1993).
There are some trial events that will likely result in an objection, motion to strike, request for curative instruction, and a motion for mistrial:
Reference to the defendant's liability insurance or insurer in testimony elicited by plaintiff's counsel, or in plaintiff's counsel's closing argument. See, e.g., Snowhite v. State, 243 Md. 291 (1963).
References to race or racial discrimination in a suit in which there is no claim for discrimination. See, e.g., Tierco Maryland, Inc. v. Williams, 381 Md. 378 (2004).
Counsel's expression of his or her own opinion on the issues in closing.
A derogatory remark about the opposing party. Renaudo v. Bloom, 209 Md. 1 (1956).
The "Golden Rule" argument in closing, inviting the jurors to step into the shoes of the plaintiff. See, e.g., Werner v. Upjohn Co., 628 F.2d 848 (4th Cir. 1980), cert. denied, 449 U.S. 1080.
Arguing matter outside the record.
Comments on failure of a witness to testify, when the witness was equally or more available to the party who is making the adverse comment.
Mention of excluded matter.
Vouching personally for a witness.
Juror misconduct, or inappropriate contact between parties or their attorneys and jurors.