Be sure to dup load paths before clearing the array.

activemodel/test/cases/validations/i18n_validation_test.rb

@@ -9,7 +9,7 @@ def setup
     Person.reset_callbacks(:validate)
     @person = Person.new
 
-    @old_load_path, @old_backend = I18n.load_path, I18n.backend
+    @old_load_path, @old_backend = I18n.load_path.dup, I18n.backend
     I18n.load_path.clear
     I18n.backend = I18n::Backend::Simple.new
     I18n.backend.store_translations('en', :errors => {:messages => {:custom => nil}})

activerecord/test/cases/validations/i18n_validation_test.rb

@@ -8,7 +8,7 @@ class I18nValidationTest < ActiveRecord::TestCase
   def setup
     Reply.validates_presence_of(:title)
     @topic = Topic.new
-    @old_load_path, @old_backend = I18n.load_path, I18n.backend
+    @old_load_path, @old_backend = I18n.load_path.dup, I18n.backend
     I18n.load_path.clear
     I18n.backend = I18n::Backend::Simple.new
     I18n.backend.store_translations('en', :errors => {:messages => {:custom => nil}})