Eh, what if it was trained on all the previous cases ever to have existed? I think it could be pretty good, as long as it detects novelty as a to flag and confirm error case.
There are other kinds of failures that will be much harder to detect: arguments that sound right but are logically flawed, lost context due to inability to read body language and tone of voice, and lack of a coherent strategy, to name a few.
All of these things could theoretically be solved individually, but each would require new systems to be added which have their own new failure modes. At our current technological level the problem is intractable, even for seemingly simple cases like this one. A defendant is better off defending themselves with their own preparation than they are relying on modern AI in the heat of the moment.