I have seen this too - it is like that part of the image just isn't rendered. Very annoying to have a grey block in the middle of someones head!  I have moved the camera position slightly and re-rendered, which can solve the problem, but sometimes at the expense of the composition you really wanted.

If I still can't get it to render an important part of my image, I save the original "faulty" render and then use the area render tool to select and render only the area of the image where the problem is occuring - mostly (not every, but certainly most times) this area will render quite happily. Save that and then I bring both images into PSP or similar as separate layers and then merge them.

Bit fiddly, but with a little practice, no one will ever know!