Revolving world, missing part of image

I watched about three minutes of that tutorial, and I agree with Mylenium -- that tutorial's a piece of crap.

Just because the guy's got a British accent doesn't make him smart.

That's one heck of a rubbish tutorial. Animating a mask in a pre-comp? SRSLY? I could think of at least three ways to do this differently without the hassle. Therein is also your answer: You are rotating the sphere too far and the mask motion is not in sync with the rotation, hence the harsh edge between the day and night side become apparent. Fix that by adjusting the mask keyframes or simply find a better tutorial that uses a better, safer technique.

Mylenium

It's either related to your texture or your settings.  You need to provide more information.  Are you using the textures maps provided by the uploader? It would be helpful to know the exact time stamp in the tutorial where your results diverge from the uploader's.  Also screen shots of your CC Sphere settings