Lion introduced a new feature, full screen mode, that allows apps to take up the entire desktop space, hiding the dock and status bar for a more iOS-like experience.
While in this mode, if you move the cursor to the top of the screen, the status bar slides back in and pushes the top of the app down slightly and an interesting thing happens: the slightly rounded corners at the top of the app get squared.
Full screen, normal:
And with the status bar:
This commitment to an aesthetic goes beyond the necessary. They could have left the corners rounded and few would have noticed.
But they didn’t. This tiny, inconsequential, practically unnoticeable detail was attended to, making the back of the cabinet as good as the front.