Adding validation to the user type to confirm that the

group list does not contain any commas. This seems to be
a common problem.

lib/puppet/type/user.rb

@@ -220,6 +220,9 @@ module Puppet
                 if value =~ /^\d+$/
                     raise ArgumentError, "Group names must be provided, not numbers"
                 end
+                if value.include?(",")
+                    raise ArgumentError, "Group names must be provided as an array, not a comma-separated list"
+                end
             end
         end
 

test/ral/type/user.rb

@@ -362,6 +362,12 @@ class TestUser < Test::Unit::TestCase
         user.delete(:groups)
     end
 
+    def test_groups_list_must_not_contain_commas
+        assert_raise(Puppet::Error) do
+            Puppet::Type.type(:user).create :name => "luke", :groups => "root,adm"
+        end
+    end
+
     def test_autorequire
         file = tempfile()
         comp = nil