Does ambiguity in law go in favor of the defendant in a criminal case? Is it an out if the law a person is accused of breaking is ambiguous or in any way poorly written that an accused can be set free from it? I cannot think that every law enacted are made equally. Sometimes laws can be rushed while some party has enough power to do so, without the necessary care and thought as to the precise practical implications of the law. Surely, there must ways of dealing with poorly written laws when a person's future is at stake?
This is known as the rule of lenity. A common formulation of the rule of lenity is as follows: If a federal criminal statute is grievously ambiguous, then the statute should be interpreted in the criminal defendant's favor. Importantly, the rule of lenity does not apply when a law merely contains some ambiguity or is difficult to decipher. As this Court has often said, the rule of lenity applies only when “ ‘after seizing everything from which aid can be derived,’ ” the statute is still grievously ambiguous. The rule “comes into operation at the end of the process of construing what Congress has expressed, not at the beginning as an overriding consideration of being lenient to wrongdoers.” Kavanaugh J., concurring, in Wooden v. United States, 595 U.S. ___ (2022), citing Ocasio v. United States, 578 U.S. 282, 295, n. 8 (2016), internal citations removed.
Not the same way as there's protection against double jeopardy in the criminal system. If Person A and Person B both have claims against Person C, even if it's for the same act or omission, each of them can independently pursue those claims. Imagine what would happen if that weren't the rule: Whoever filed first would functionally be preventing the other one from recovering their claim. What's worse, Person A and Person C could collude (say, by presenting a bad case on Person A's side) and prevent person B from accessing relief that they're entitled to. The way that the courts prevent abuse of the system by people who want to keep re-litigating the same issue is a principle called res judicata. But that's a principle that only applies when it's the same parties – say, Person A sued Person C and lost, and so sued Person C again for the same claim. This limitation protects Person B against any collusion or just bad lawyering on the part of Person A.
Perjury is not 'not telling the truth'. It requires (in most jurisdictions) being proven to have deliberately lied under oath. As Xavier pointed out, you are not on oath when entering a plea (among other reasons, you would be required to incriminate yourself). Secondly, "I am not guilty" could mean anything from "the prosecution wrongly think that what I did is illegal" to "this law is unconstitutional" even if the facts are not in dispute; either may be mistaken without being a lie. If the facts are in dispute, the jury will have to deliver a verdict that implies (it does not state, let alone prove) that they disbelieve one party; deliberate falsehood, whether by a defendant or a police officer, would be several steps beyond that.
See jury-nullification. I'm not a legal historian, so I can't say for sure what the laws on jury acquittals were at that time in that jurisdiction. However, when a jury has final discretion to acquit a defendant of a crime that's it: They can effectively ignore laws if they want to acquit someone. Such acquittals do not set a precedent or have any bearing outside of the trial in which they issue their verdict.
If the prosecutor (P) knows or strongly believes based on this new evidence that A is innocent, ethically P should start proceeding to have A's conviction reversed or reviewed. But in far too many cases P does nothing of the sort. If P simply files a charge against B and proceeds to try B for the crime, P leaves it open to B's Lawyer to ask &quot;Didn't you already convict A for this crime? what about that?&quot; as part of a defense, which might well embarrass P and lose the case. So P may well choose to file charges against B claiming that A &amp; B acted together as accomplices, even if this requires misstating the evidence, or suppressing part of it. Or, P may simply ignore the new evidence, leaving A in prison and B free. This is unjust, but requires no effort on the part of P, and may seem less likely to raise embarrassing questions about why P got the case against A wrong. P can always claim that s/he did not believe the new evidence. That might even be true, there is such a tendency to believe what we wish to believe. The relative frequency of these responses on the part of those in the position of P here is really not possible to asses. The last two responses involve P suppressing or at least burying relevant evidence, and unless it is brought to the attention of others who publicize it enough that action is taken, it will not be generally known and cannot be tabulated in any statistics. P's office will certainly not respond to any survey which asks &quot;How many times this year did you suppress the true facts to leave in place an unjust conviction you had previously obtained?&quot;
This is not possible, simply as a matter of definitions and legal terminology. Someone who is "represented by one or more attorneys" is by definition not pro se which means representing yourself without an attorney. There are very rare instances in criminal trials involving serious consequences in which a pro se defendant is allowed to have an attorney advisor who does not represent them in court in an agency capacity, but, first, people who do that almost always lose and are almost always mentally ill (although not necessarily eligible for an insanity defense), and second, because courts generally don't allow this in any other circumstance (at least in court). The concept of getting advice from an attorney without having full fledged representation is called a "limited representation" and the law regarding limited representations more generally varies greatly from jurisdiction to jurisdiction and even between different courts in the same place. For example, Colorado's state courts and Colorado's federal courts have different rules for limited representations.
I'm no expert, but I had assumed this clause was present in case of the following situation. Joe is arrested for a robbery of a London bank. Joe says nothing under questioning. At trial, Joe's defense is that at the time of the robbery, he was in Sheffield drinking beer with his brother. On the basis of common sense, a jury could think: "Surely if Joe were really innocent, he would have told the police of his alibi at the time he was questioned, and saved himself a lot of trouble. Since he didn't do that, maybe a more likely explanation is that he wasn't actually in Sheffield, but that sometime between arrest and trial, he came up with the idea of faking an alibi in Sheffield, and convinced people to testify falsely to that effect. Yes, that does seem more plausible. So we are not going to give much credence to Joe's supposed alibi." So it really would be the case that not mentioning the alibi during questioning would harm Joe's defense at trial. The warning, then, is intended to keep Joe from doing this inadvertently. If Joe's alibi is genuine, but out of a misguided desire to exercise his right to remain silent, he doesn't mention it during questioning, he may accidentally increase his chances of being wrongly convicted. Everybody has an interest in avoiding this.
In the United States, the constitutionality of a law can be challenged in any court. Unless and until the court is reversed by a higher court, then the law will be considered unconstitutional and invalid. A jury can also theoretically nullify a law for any reason, including unconstitutionality, however, the jury's finding will only hold for that one case that they are trying.