Failing to mention when questioned something you later rely on in court
<p>If you are arrested in the UK, you'll hear that your rights include a rider inserted by the 1994 Criminal Justice and Public Order Act, after "you do not have to say anything":</p>

<blockquote>
  <p>But, it may harm your defence if you do not mention when questioned something which you later rely on in court.</p>
</blockquote>

<p>In common with, I think, many British laymen, I have never understood the justification for this change, and have only ever seen it as vaguely threatening and a non-specific erosion of civil liberties. At the same time, I'm not really sure what specific sort of abuse I have to fear from it.</p>

<p>Can anyone provide me with understandable, concrete examples of</p>

<ol>
<li><p>Why (if at all) I should be thankful that this clause exists? i.e., What kind of criminal act might a criminal get away with if this rider weren't there?</p></li>
<li><p>What I have to fear? i.e., (a) What's the worst that might happen to a distrusting but innocent person who is wrongfully arrested and decides to say nothing? (b) How might the negative consequences have been avoided by saying something? (c) What additional risks are entailed in opening one's mouth in this situation, and how can they be minimized?</p></li>
</ol>

<p><strong>EDIT</strong>: having read <a href="https://law.stackexchange.com/a/7041/4212">Nate's interpretation</a> and Dawn's comments, I can re-frame the question more specifically.  Either <strong>A</strong>: there was a simultaneous change in the way a suspect's silence could be <em>used or interpreted</em> in an English/Welsh court (away from the US model in which prosecutors cannot even draw attention to it), and the change in wording reflects this erosion of civil liberties;  or <strong>B</strong>: both before and after the Criminal Justice Act, silence could always harm your defense, and the change in wording is an entirely benevolent attempt to warn people of this fact. Can anyone tell me which is the case?</p>

<p>I'm no expert, but I had assumed this clause was present in case of the following situation.</p>

<p>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.</p>

<p>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."</p>

<p>So it really would be the case that not mentioning the alibi during questioning would harm Joe's defense at trial.</p>

<p>The warning, then, is intended to keep Joe from doing this <em>inadvertently</em>.  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.</p>

<p>Your point <strong>A</strong> is exactly right.  The 1994 Act allowed the prosecution to draw attention to the fact that you refused to say "but I was with Jack the Knife all the time" at the time you were initially questioned.</p>

<p>Prior to that, the prosecution could not mention that a question was asked which you did not answer.</p>

<p>This was, in my view, a serious erosion of civil liberties, and I decided at the time that I would refuse to answer any questions under caution as a protest.  (I have never, before or since, been questioned under caution).</p>

united-kingdom criminal-law human-rights arrest self-incrimination