The cassandra test cluster is still on trusty, which has different versions of cassandra and other packages. This causes occasional incompatibilities in the configs. For example, right now the production cluster runs cassandra 2.1.3, while the test cluster is stuck at 2.1.2. The version 'jamm.jar' has changed between those versions, which prevents 2.1.2 from starting up with the 2.1.3 config.
It would be great to upgrade the test boxes to Jessie too, so that we can avoid working around incompatibilities & have more reliable test results before deploying changes to production.