This expression would match more than just "puppet".

Example:

root@eggplant:~/infra-puppet# gem list | grep puppet
librarian-puppet (0.9.9)
root@eggplant:~/infra-puppet# gem uninstall -ax puppet facter
ERROR:  While executing gem ... (Gem::InstallError)
    cannot uninstall, check `gem list -d puppet`

local-modules/base/manifests/init.pp

@@ -149,7 +149,7 @@ class base::post {
 
   exec {
     'kill puppet gems' :
-      onlyif  => 'gem list | grep puppet',
+      onlyif  => 'gem list | grep ^puppet',
       command => 'gem uninstall -ax puppet facter',
       path    => ['/var/lib/gems/1.8/bin', '/usr/local/bin', '/usr/bin', '/bin'];
   }