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Building with a new SDK makes the app opt in to new behaviors even when targeting older iOS versions. This can be quite a hassle. Your perfectly working app - even when deployed on an iOS 8 device - will suddenly break in many fun ways merely from recompiling with a newer SDK: touch handling differs, margins may be weird, table headers vanish because various new methods aren't implemented, screen rotation suddenly gets enabled where you didn't expect, new keyboard metrics mess up your layout, etcetc. This happens because when the runtime detects that an app is built with an older SDK, it enables a bunch of compability behaviors that newly built apps cannot opt in to.