The documentation on which version of Casbah supports which version of the Java driver is missing from the http://docs.mongodb.org/ecosystem/drivers/scala/ page. According to the changelog, Casbah is currently incompatible with 3.0.3. http://mongodb.github.io/casbah/changelog.html.

We need to update the Scala Driver page to clarify what version of the Java driver is supported by each version of Casbah.