I have a managed object with a dueDate attribute. Instead of displaying using some ugly date string as the section headers of my UITableView I created a transient attribute called "category" and defined it like so:

- (NSString*)category
{
    [self willAccessValueForKey:@"category"];

    NSString* categoryName;
    if ([self isOverdue])
    {
        categoryName = @"Overdue";
    }
    else if ([self.finishedDate != nil])
    {
        categoryName = @"Done";
    }
    else
    {
        categoryName = @"In Progress";
    }

    [self didAccessValueForKey:@"category"];
    return categoryName;
}

Here is the NSFetchedResultsController set up:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task"
                                          inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

NSMutableArray* descriptors = [[NSMutableArray alloc] init];
NSSortDescriptor *dueDateDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dueDate"
                                                                  ascending:YES];
[descriptors addObject:dueDateDescriptor];
[dueDateDescriptor release];
[fetchRequest setSortDescriptors:descriptors];

fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Root"];

The table initially displays fine, showing the unfinished items whose dueDate has not passed in a section titled "In Progress". Now, the user can tap a row in the table view which pushes a new details view onto the navigation stack. In this new view the user can tap a button to indicate that the item is now "Done". Here is the handler for the button (self.task is the managed object):

- (void)taskDoneButtonTapped
{
    self.task.finishedDate = [NSDate date];
}

As soon as the value of the "finishedDate" attribute changes I'm hit with this exception:

2010-03-18 23:29:52.476 MyApp[1637:207] Serious application error.  Exception was caught during Core Data change processing: no section named 'Done' found with userInfo (null)
2010-03-18 23:29:52.477 MyApp[1637:207] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'no section named 'Done' found'

I've managed to figure out that the UITableView that is currently hidden by the new details view is trying to update its rows and sections because the NSFetchedResultsController was notified that something changed in the data set. Here's my table update code (copied from either the Core Data Recipes sample or the CoreBooks sample -- I can't remember which):

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[self.tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            // Reloading the section inserts a new row and ensures that titles are updated appropriately.
            [self.tableView reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}

I put breakpoints in each of these functions and found that only controllerWillChange is called. The exception is thrown before either controller:didChangeObject:atIndexPath:forChangeType:newIndex or controller:didChangeSection:atIndex:forChangeType are called.

At this point I'm stuck. If I change my sectionNameKeyPath to just "dueDate" then everything works fine. I think that's because the dueDate attribute never changes whereas the category will be different when read back after the finishedDate attribute changes.

Please help!

UPDATE:

Here is my UITableViewDataSource code:

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[self.fetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil)
    {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }

    [self configureCell:cell atIndexPath:indexPath];    

    return cell;
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];    
    return [sectionInfo name];
}