The grounds for a mistrial in a civil case in California fall into four main categories: juror misconduct, misconduct by an attorney or the judge, a prejudicial incident or serious evidentiary error, and a deadlocked jury. A judge declares a mistrial when something has gone so wrong that no instruction to the jury can repair the damage. It ends the trial before a verdict, wipes the slate clean, and sends the case back for a new trial with a new panel. Because it wastes weeks of work and money on both sides, California courts treat it as a last resort.
Juror Misconduct
Juror misconduct is one of the most common triggers. California’s Code of Civil Procedure lists “misconduct of the jury” as a standalone ground for vacating a verdict and ordering a new trial, and the same principle applies mid-trial: if a juror’s behavior is so damaging that continuing would be pointless, the judge can stop the proceedings.1California Legislative Information. California Code CCP 657 – Grounds for New Trial
Judges are required to instruct jurors not to research the case, share information about it, or discuss the trial with anyone. That admonition explicitly covers electronic and wireless communication, so social media posts and internet searches fall within the prohibition.2California Legislative Information. California Code CCP 611 – Admonitions to Jury
Common forms of juror misconduct include:
- Outside research on the case, the parties, or the legal issues, which introduces information that was never tested through cross-examination or ruled admissible.
- Unauthorized contact with a witness, attorney, or party outside of open court.
- Concealing a personal or financial relationship during jury selection that would prevent impartiality.
- Being intoxicated or under the influence during trial proceedings.
Proof has its own rules. California allows evidence of what a juror said or did, inside or outside the jury room, as long as the conduct was likely to improperly influence the verdict. The law draws a firm line at the juror’s internal thought process: no one can introduce evidence about how a particular statement affected another juror’s decision.3Justia Law. California Code Evidence Code 1150 – Evidence to Impeach Verdict You can show that a juror searched the defendant’s name online. You cannot ask another juror to testify about how that search changed their thinking.
Attorney and Judicial Misconduct
Attorneys who repeatedly cross the line can force a mistrial. The usual scenarios involve inflammatory statements in opening or closing that have no basis in the evidence, or persistent attempts to introduce testimony or exhibits the judge has already excluded. One slip is often fixable with a jury instruction. A pattern can poison the proceedings beyond repair.
California’s grounds for a new trial capture this through two categories: “irregularity in the proceedings of the court, jury or adverse party” that prevented a fair trial, and “error in law, occurring at the trial.”1California Legislative Information. California Code CCP 657 – Grounds for New Trial Both apply to attorney behavior and judicial error alike.
The judge’s own conduct can also require a mistrial. Comments in front of the jury that reveal a clear bias toward one side, or legally incorrect instructions that would push the panel to apply the wrong standard, can create prejudice that no correction will undo. When that happens, the remedy is the same as for any other form of incurable prejudice: start over.
Prejudicial Incidents and Evidentiary Errors
Not every mistrial follows intentional wrongdoing. Sometimes an unexpected event contaminates the trial despite everyone’s best efforts. A witness blurts out something the jury was never supposed to hear. A spectator has an emotional outburst that changes the atmosphere in the courtroom. These are nobody’s fault in the traditional sense, but they can be just as damaging as deliberate misconduct.
A classic example involves liability insurance. California law makes it inadmissible to mention that a defendant carried insurance against the type of harm at issue, because that information could tempt a jury to find liability simply because an insurer would foot the bill.4California Legislative Information. California Code Evidence Code 1155 – Evidence of Liability Insurance If a witness casually mentions the defendant’s coverage on the stand, the cat is out of the bag, and depending on how the statement registered with the jury, a curative instruction may not be enough.
Serious errors in the judge’s evidentiary rulings can also warrant a mistrial. If a key piece of evidence is improperly admitted or wrongly excluded and the ruling effectively prevents one side from presenting its case, the error may qualify as an “irregularity in the proceedings” that prevented a fair trial.1California Legislative Information. California Code CCP 657 – Grounds for New Trial These situations are rare, because judges have broad discretion on evidence, but a bad call on a critical issue can derail everything.
Deadlocked Jury
A hung jury is the most straightforward path to a mistrial. When jurors cannot agree after extensive deliberation, the judge has to end the trial. California civil cases do not require a unanimous verdict. Three-fourths of the jury must agree, which means at least nine of twelve jurors need to be on the same side.5California Legislative Information. California Code CCP 618 – Verdict
Before declaring a mistrial for deadlock, the judge will typically send the jury back to keep deliberating, sometimes more than once, to be satisfied that the panel has genuinely exhausted every chance of agreement. Once it is clear that no additional discussion will get nine jurors to align, the judge declares a mistrial and discharges the panel. California law expressly allows the case to be retried immediately or later once the jury is discharged without a verdict.6Justia Law. California Code CCP 616 – Discharge of Jury
Why Curative Instructions Usually Come First
Judges do not jump to a mistrial every time something goes wrong. It is a drastic remedy, and most judges will first try a curative instruction, telling the jury to disregard the offending statement, evidence, or conduct when deciding the case.
Curative instructions work best when the prejudicial event was isolated and unintentional. A prompt, specific instruction to ignore an accidental slip will often be enough. They fall short when the prejudice is severe or repeated. A single offhand remark about insurance might be curable. A detailed description of the defendant’s policy limits probably is not. The more vivid and memorable the improper information, the less realistic it is to expect jurors to simply forget it.
This is where mistrial motions succeed or fail. The moving party has to convince the judge that no instruction can undo the damage. If the judge believes the jury can follow a curative instruction and still deliver a fair verdict, the motion is denied.
How a Mistrial Motion Works
A mistrial does not happen automatically. In most cases, an attorney has to request one by making a motion for a mistrial. Timing matters. The motion should be made as soon as possible after the prejudicial event, because waiting looks like a tactical decision to see how things play out, which courts treat as giving up the right to complain.
The motion is made orally, outside the hearing of the jury. The attorney identifies what happened, explains why it was prejudicial, and argues that the harm cannot be cured with an instruction. The other side responds, usually by arguing that the event was not as prejudicial as claimed or that a curative instruction would solve it. The judge rules on the spot.
A judge can also declare a mistrial without a motion from either party. This power comes up most often with a deadlocked jury, but it applies whenever the judge observes something that fundamentally compromises fairness. Judges use it sparingly, since the decision to start over carries significant strategic and financial consequences for both sides.
What Happens After a Mistrial
When the judge declares a mistrial, the trial stops and the jury is dismissed. A mistrial is not a win or a loss. It does not resolve any part of the underlying dispute. The case reverts to its pre-trial status, and the lawsuit remains alive.
The main consequence is a retrial with a new jury. California law gives the parties three years from the date the mistrial order is entered to bring the case back to trial.7California Legislative Information. California Code CCP 583.320 – Time Limits for New Trial Miss that deadline and the case can be dismissed entirely.
The financial reality is significant. Each side bears its own costs for the retrial: new jury fees, witness expenses, attorney preparation time, and court reporter fees. For plaintiffs paying attorneys on contingency, the lawyer invests more hours without additional compensation unless the case eventually settles or produces a verdict. For defendants paying by the hour, the meter keeps running. That expense and uncertainty is why many cases settle after a mistrial rather than head into a second trial. Both sides have now seen the other’s strategy, witnesses, and evidence, which often makes realistic settlement talks more productive than they were before.