Rendering issue

Encountered rendering issues (as per attached screenshot)

 

Sequence of events:

1) Gone up to a very high level (didn't took note of the level). I could see a spherical world

2) Came back down

3) And parts of the image couldn't render

4) I even navigated around but still no rendering of a particular patch

5) Then switched to another application using alt+tab and then resumed game

6) The missing patch was rendered successfully