title: "[something copyWithZone:]: unrecognized selector sent to instance" when using Bindings / Core Data
body: <p>(self asking and self-answering because I spent hours on the web looking for this, and most of the resources all say "I solved it in the end" without giving an explanation)</p>
  
  <p>I had a very simple Core Data + Bindings application:</p>
  
  <ul>
  <li>An NSArrayController pulling items out of Core Data</li>
  <li>An NSTableView rendering them</li>
  <li>Another NSTableView that when you click on a row in the first table, displays the details of that item</li>
  </ul>
  
  <p>Item 3 above was causing application crash, with the error:</p>
  
  <pre><code>[(my NSManagedObject) copyWithZone:]: unrecognized selector sent to instance
  </code></pre>
  
  <p>Implementing that method (!) and putting a breakpoint there, I found it was being invoked by Apple's NSCell class - this didn't much help :(.</p>