title: Core Data invalid number of rows error without inserting/removing rows
body: <p>I'm modifying (but not adding or removing) objects in a Core Data store upon selection of a UITableViewCell. However, whenever I do so, I get the following error:</p>
  
  <pre><code>CoreData: error: Serious application error.  An exception was caught from the delegate of 
  NSFetchedResultsController during a call to -controllerDidChangeContent:.  Invalid update: 
  invalid number of rows in section 0.  The number of rows contained in an existing section 
  after the update (5) must be equal to the number of rows contained in that section before 
  the update (5), plus or minus the number of rows inserted or deleted from that section (1 
  inserted, 0 deleted) and plus or minus the number of rows moved into or out of that 
  section (0 moved in, 0 moved out). with userInfo (null)
  </code></pre>
  
  <p>...except that I've not inserted or deleted any new rows, I've just modified a property on an existing object which backs up the row. How can I fix this error?</p>