The hallucinations in legal briefs get really out of hand when the attorney wants to make an argument not supported by the case law. The LLM wants to do a good job defending a case, so it invents the legal precident, because otherwise it'd be impossible to make the argument credibly. This invites a rule 11 challenge from the other side where you claim the lawyer is so full of crap with his claim that he deserves sanction for not understanding the law and wasting everyone's time.

What's interesting about the rules of civil procedure is that it has been built up over centuries to prevent all kinds of abuse by sneaky, clever, unscrupulous litigants. Most systems are not so hardened against bad faith actors like the legal system is and AI just thinks it can pathologically lie its way through cause most people trust somebody who sounds authoritative.