Here is a very simple scene I've been working on where you can see that the renderer is not correctly rendering some of the fake stones. There is a clear cutoff between render tiles where on one tile the stone is rendered but on another tile it is not. I know that this can be a problem when working with some extreme displacements, but there is nothing extreme about this file. I've marked some of the main problem areas out in the attached image. The .tgd is also attached, but there is honestly nothing strange going on in the .tgd.
Doing crop renders of all the problem areas does work to fill in the bad sports, but then there is a great deal of extra work involved, and if this was for an animation that would be a huge problem.
Rendered in Windows Vista 64 - Q9450 - 8GB Ram so I would really not expect that this would be any sort of memory issue.
Rendered at:
Detail: 1
AA: 14
GI Relative Detail: 3
GI Sample Quality: 4
Supersample prepass on
GI Surface Details on