This can be caused by 2 things
- Compiling with fast lighting
- Turning all sides of a brush into displacements, instead of only the visible ones (the extra hidden faces sharing those edges confuse the lighting process)
If you did #2, just go back to the face edit dialog, displacement tab, then select the hidden faces and click Destroy. You can pick individual brush faces to turn into displacements in a similar way.
EDIT: some more info that might be helpful
Destroying all displacements from a single brush will revert it back to a world brush, and the
button will allow you to see the non-displacement/destroyed faces so you can turn them back.