Before creating a content setting, check that you actually *can* create it.

chrome/browser/gtk/collected_cookies_gtk.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/gtk/collected_cookies_gtk.h"
 
 #include "app/gtk_util.h"
 #include "app/l10n_util.h"
 #include "chrome/browser/cookies_tree_model.h"
 #include "chrome/browser/gtk/gtk_util.h"
@@ skipping 248 matching lines @@
                                         ContentSetting setting) {
   GtkTreeModel* model;
   GList* paths =
       gtk_tree_selection_get_selected_rows(selection, &model);
   for (GList* item = paths; item; item = item->next) {
     GtkTreeIter iter;
     gtk_tree_model_get_iter(
         model, &iter, reinterpret_cast<GtkTreePath*>(item->data));
     CookieTreeOriginNode* node = static_cast<CookieTreeOriginNode*>(
         adapter->GetNode(&iter));
-    if (!node->CanCreateContentException()) {
+    if (node->CanCreateContentException()) {
       node->CreateContentException(
           tab_contents_->profile()->GetHostContentSettingsMap(), setting);
     }
   }
   g_list_foreach(paths, reinterpret_cast<GFunc>(gtk_tree_path_free), NULL);
   g_list_free(paths);
 }
 
 void CollectedCookiesGtk::OnBlockAllowedButtonClicked(GtkWidget* button) {
   AddExceptions(allowed_selection_, allowed_cookies_tree_adapter_.get(),
@@ skipping 17 matching lines @@
   g_signal_handlers_block_by_func(
       tree_view, reinterpret_cast<gpointer>(OnTreeViewRowExpandedThunk), this);
   gtk_tree_view_expand_row(GTK_TREE_VIEW(tree_view), path, TRUE);
   g_signal_handlers_unblock_by_func(
       tree_view, reinterpret_cast<gpointer>(OnTreeViewRowExpandedThunk), this);
 }
 
 void CollectedCookiesGtk::OnTreeViewSelectionChange(GtkWidget* selection) {
   EnableControls();
 }