What can be used against you in a divorce?
Almost everything that hurts people is something they did after the case began, and most of it was self-inflicted in the first few weeks. Courts are much less interested in who behaved worse during the marriage than in conduct that affects money, children, or the honesty of the process.
That reframing is the useful part. You have limited control over the history and near-total control over the next month.
The four categories
Things you wrote. Texts, emails, and posts, which are ordinary evidence and are read aloud in a tone you did not intend. Messages and screenshots and social media each have their own rules, and the same one underlies both: assume a judge will read it.
Things you did with money. Moving funds out of joint accounts, large purchases, new debt, income that suddenly drops, accounts nobody mentioned. Financial conduct gets scrutinized closely because it is documented by third parties who have no stake in your marriage.
Things you did with the children. Withholding them, questioning them about the other household, relaying messages through them, disparaging their other parent within earshot. This is the category that does the most lasting damage, both to the children and to how a judge reads you.
Things you did unilaterally. Changing the locks, taking the children out of state, canceling insurance, enrolling a child in a new school without consent. The pattern courts dislike is not the act so much as the deciding alone. Two of those can be more than a bad look: in the states with automatic restraints, canceling insurance or taking the children out of state can breach a standing order, and a lockout is not something you can impose without one. Those orders do leave room for ordinary life. They generally let you keep paying the household bills and let you use money to hire a lawyer. What they want is warning before anything unusual: California, for one, asks you to tell your spouse five business days ahead of any out-of-the-ordinary spending and to account for it to the court afterward. Read the order that came with your papers rather than guessing at what it covers.
The multiplier
Any of the above gets much worse if you are not candid about it.
Incomplete disclosure, a document that turns out to be edited, a claim contradicted by your own bank records: these do more harm than the underlying facts. A judge who concludes you shaded the truth once discounts everything else you say, including the parts that were true and helpful.
Being the person whose disclosure was clean is worth more than anything concealment could protect. That holds even when the honest version is unflattering.
What matters much less than people fear
An affair, in most states, as far as property and custody go. Every state offers a no-fault route, and infidelity usually has limited effect on either unless money was spent on it or it affected the children directly. Around thirty states do still allow fault to be pleaded, and in more than half of the states conduct is one of the things a judge may weigh in spousal support, so it is worth asking about yours. It matters enormously to you and often very little to those parts of the outcome.
Spousal support is the exception, and it is a large one. A handful of states make adultery a statutory bar to alimony rather than merely a factor: in South Carolina an adulterous spouse cannot be awarded alimony at all, and North Carolina makes the effect mandatory in both directions. Even there, do not concede the point on your own. In North Carolina the rule turns back into the judge's discretion if both of you strayed, and conduct your spouse forgave, by taking you back after learning of it, cannot be counted against you at all. Other states exclude misconduct evidence entirely. If support is in play in your case, confirm your own state's rule rather than assuming fault is irrelevant.
Being the one who left. Moving out does not forfeit your interest in the house, and it is not treated as abandonment in the way people fear. The specifics are here.
Ordinary imperfection. A messy house, a missed appointment, a poor month. Courts are not choosing the better person; they are dividing a life and arranging care for children.
If something already happened
Tell your attorney, early and plainly, including the parts you suspect were mistakes. Nearly everything is more manageable raised early than discovered later by the other side, and a lawyer who is surprised in a hearing cannot protect you.
The full list of instincts worth resisting is in the mistakes that cost people most. Its underlying rule is the same as this page's: put a night between an impulse and an action, and most of this never happens.
How much weight fault carries, and what conduct affects property or custody, vary by state. If something on this page has already happened in your case, that is a conversation for a licensed family-law attorney rather than a reason to keep quiet.