A while ago, I somehow ended up enrolling my laptop on the Mac OS X Beta program, receiving updates in advanced.  This was all going great, until I got an error on iTunes Connect saying

New apps and app updates must be built with the public (GM) versions of Xcode 6 or later, OS X, and iOS SDK. Don’t submit apps built with beta software including beta OS X builds.

Oh no – my Mac had done an automatic update to a Beta release, and this in turn meant that my laptop was unable to compile apps for iTunes Connect / App Store.

Moral of the story:  Turn off automatic beta updates if you are developing using Xcode.