title: can't add a new key into hash during iteration
body: <p>Can anyone explain to me what exactly mean this runtime exception?</p>
  
  <pre><code>can't add a new key into hash during iteration
  </code></pre>
  
  <p>I have this error in this piece of code (Im using Mongoid):</p>
  
  <pre><code>new_item_entries = []
  
  item.entries.each do |entry|
    new_item_entries &lt;&lt; ItemEntry.new(name: entry.field) # exception here
  end  
  
  self.entries = new_item_entries
  self.save
  </code></pre>
  
  <p>But this exception burns in NOT any cases! In my Rspec test I have 3 of 70 test fails with this exception. Most of tests use code above.</p>