I have a project that was working just fine with XCode 5 and iOS 7, but when I recently downloaded XCode 6, I noticed some weird behavior with autolayout.
For me it was an issue with the tab bar in my project. I didn't need it in that view and I hid it on push. So when the view got pushed onto the stack the bar was hidden and the constraints adjusted accordingly. So I just put the tab bar back in.