We have seen it many many times on the forum: People are linking a CSG bursh to another entity script, but the script fails on startup because the linked in entity is nil. The reason: the CSG brush does not have mass or an entity script attached and is being collapsed at the start of the scene for performance.
http://www.leadwerks.com/werkspace/topic/15761-entity-gets-has-stopped-working-error/
This 'collapsing' has caused a lot of headache for a lot of people. People often don't know that CSG collapses when the scene starts and what the result of this is. Also, the workaround for setting a mass or attaching a script is still unknown by a lot of people.
Since people are not aware of the technical reason of the CSG collapsing, I would say, make sure that linked CSG brushes are not collapsed. Even if they do not contain a script or have a mass.