Some other potential cans of worms: I think barring true psychosis*, some questions would be pretty straightforward: “Did you kill him?” “Did you rob the First Planetary Bank of Mallorysport?” and so forth.
*And if the defendant was in fact observed to commit the crime, and was then immediately apprehended, but as soon as he’s taken to the police station he’s all “Wh-where am I? What happened? Oh, God! Why is there blood on my hands?!?”, it would be morally and legally useful to be able to actually verify that he isn’t just faking it, he really DID have a psychotic break / has “multiple personalities” / had been mind-controlled by an evil telepath.
But if we get in to questions not just of objective actions, but of motives and state of mind, I suspect things could get a lot murkier. I wouldn’t be all that surprised if someone might be able to “blue light” the machine when claiming “He was coming right for me! I was in fear for my life! It was self-defense! I had no choice!” even if a videotape might show something quite different, or at least pretty ambiguous. Especially if the veridicated testimony is made some time after the events, allowing the accused plenty of time to repeatedly go over everything in his own mind and mentally work his way around to a version of events that puts him in the best possible light.
Even worse, I can imagine this process might work against basically decent people, and in favor of narcissistic jerks. Suppose Conscientious Charlie and Trigger-Happy Ted are both accused of murder or manslaughter, but in both cases it might have been self-defense.
Conscientious Charlie:
“Did you kill him?” “Yes, I did.”
“But it was self-defense, right?” “It–I--yes, it was self-defense.” (Oh, god, I killed that man! Was that really the only thing I could have done under those circumstances?)
“But you had no choice, right? You had to kill him to defend yourself against an attack you feared might result in your own death or grievous bodily harm, right?” “I–Yes! No! I don’t know!”
But the videotape (if there is one) shows poor Charlie acted totally reasonably.
Meanwhile, Trigger-Happy Ted (the narcissistic jerk):
“Did you kill him?” “Yes, I did.”
“But it was self-defense, right? You had no choice, right? You had to kill him to defend yourself against an attack you feared might result in your own death or grievous bodily harm?” “You got it, Jack.”
But the videotape shows the circumstances were much more murky than that, or even that Ted really wasn’t at all justified.
I can also imagine a case where Bob says “Sure, we had sex–it was totally consensual!” and Alice says “I didn’t consent! He forced himself on me!”, and then where are you? And again, poor Conscientious Charlie thought it was a consensual and mutually pleasurable encounter, but when he hears Alice is accusing him of rape, he starts second-guessing himself, and soon he’s red-lighting all over the place (or blue-lighting to “I…Oh, God!..I must have raped her! I’m so sorry!”) even though a videotape might show Alice practically dragging Charlie into the bedroom and (with his full consent at the moment) having her way with him. Again, a narcissist or sociopath might be at an advantage in that sort of situation against a basically decent person.