In 3.0 we introduced the listCollections command and an associated privilege to run it, replacing the old way of listing collections of querying system.namespaces directly. The problem is that mongoses load what privileges a user should have from the config servers, so if the config servers are still running 2.6, they will not provide the listCollections privilege. This means that future listCollections requests to a 3.0 mongos will fail with an "unauthorized" error.