Saddle Rail Failure?
The first issue with carbon rail seat posts is the lack of a standard for rail diameters.  This often leads to the seat post clamp being improperly sized when compared to the saddle rail.  

Due to the stress riser of a seat post clamp on carbon, an ill fitting seat post clamp can contribute un-necessary stress to the carbon rails.

Carbon is much more likely to develop stress fractures due to over tension of a seat post clamp\stem clamp\etc than aluminum or steel.  Because of this, and the above situation, proper tension of the seat post clamp (along with an anti friction paste) is paramount.

Personally I don't think standard carbon rail seat posts are worth the hassle (of fitting them propertly, tensioning them properly, etc) of saving 20-40 grams.  Double so if you are doing any type of off road riding (like cross) where the external forces at play are magnified.