Why does UICollectionView log an error when the cells are fullscreen?

There is a property on UIViewController–automaticallyAdjustsScrollViewInsets–that defaults to YES. This means that when a UIViewController has a UIScrollView in its view hierarchy–which is true of a UICollectionViewController–the contentInset property of that scroll view is adjusted automatically to account for screen areas consumed by the status bar, navigation bar, and toolbar or tab bar.

The documentation for that property states:

automaticallyAdjustsScrollViewInsets

Specifies whether or not the view controller should automatically adjust its scroll view insets.

@property(nonatomic, assign) BOOL automaticallyAdjustsScrollViewInsets

Discussion

Default value is YES, which allows the view controller to adjust its scroll view insets in response to the screen areas consumed by the status bar, navigation bar, and toolbar or tab bar. Set to NO if you want to manage scroll view inset adjustments yourself, such as
when there is more than one scroll view in the view hierarchy.

The solution is to set automaticallyAdjustsScrollViewInsets to NO somewhere in your UICollectionViewController subclass, such as in viewDidLoad:

- (void)viewDidLoad {
    [super viewDidLoad];
    self.automaticallyAdjustsScrollViewInsets = NO;
}

I have put an example project on GitHub that illustrates this problem and solution. There are two branches: with_error and fixed_error. Here is a diff of the change on GitHub.