happening to me when I attempted to container:start (from xsbt-web-plugin) within an sbt session on a Spray/spray-servlet application. I was going to great lengths to search my $CLASSPATH and dependency's transitive dependencies to determine what was relying on an older version of Netty causing the Datastax driver to fail. I was coming up empty handed.
Then I ran the application from IntelliJ and it worked just fine. This meant either my shell or sbt session was suspect.
My default $CLASSPATH variable is empty so I figured it probably wasn't my shell.
Executing show fullClasspath from within an sbt session gave me a ton of paths but only the expected netty dependency.
Unfortunately modifying and updating the source for xsbt-web-plugin to exclude("org.jboss.netty", "netty") - this was a much older version of netty bundled, before the change to the "io.netty" organization - didn't seem to fix the problem.