I am not sure if this behaviour is expected or is it a random bug in XCode 6.
* When you move from one view controller to another using new "show" segue(it presents the content in the detail or master area depending on the content of the screen), then you cannot add UIBarButtonItem on the navigation bar. 
When you try to do this, the Bar Button Item drops to the bottom, and in fact doesn't show up when I build the project.
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* But when you have a View Controller in a navigation stack and you use the old "push" (deprecated segue) instead of new "show" segue, then you can drag a UIBarButtonItem to the Navigation Bar.
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So if you really want to use UIBarButtonItem with new "show" segue, then you can use following hack:
Set it up as 'Push' to begin with, drag on the UIBarButtonItem, and then change the segue to 'Show'.