Since version 4.0.9 of the Karaf container the feature pax-web (org.ops4j.pax.web.pax-web-features) contains a list of jetty dependencies.
The jetty-deploy bundle is missing.
This bundle is needed for example to make configuration to support context xml deployment for enabling static resources.
The following configuration in the jetty.xml file will throw a class not def. error on the start of Karaf (or jetty)
A possible fix is to add the missing dependency in the file features.xml of the pax-web feature definition (for example in pax-web 4.3.0 for karaf 4.0.9 and in pax-web 6.0.3 for karaf 4.1.1)