Maven error "Cannot find wagon which supports the requested protocol: ftp" and Hudson

It is important to understand that Hudson comes bundled with Maven but you can also configure Hudson to use an external installation of Maven (Manage Hudson > Configure System > Maven 3 section).  When you add a Maven build step to your Job, you have the option to use the bundled Maven or your external installation.  However, the default is the bundled Maven. 

This can cause problems if you need additional classes required by your Maven plugins that are not automatically downloaded, e.g. FTP support for the Deploy Plugin or Wagon Plugin (http://mojo.codehaus.org/wagon-maven-plugin/).  That is, if you have dropped the required JAR into the "lib" sub-folder of your external Maven installation but the Job is configured to use the bundled Maven, you will continue to get "Cannot find wagon which supports the requested protocol: ftp" (in this case).