diff --git a/wp-admin/categories.php b/wp-admin/categories.php
index 208fe26a64..d38cd0c1ff 100644
--- a/wp-admin/categories.php
+++ b/wp-admin/categories.php
@@ -13,7 +13,7 @@ $title = __('Categories');
 
 wp_reset_vars(array('action', 'cat'));
 
-if ( $_GET['action'] == 'delete' && isset($_GET['delete']) )
+if ( isset( $_GET['action'] ) && $_GET['action'] == 'delete' && isset($_GET['delete']) )
 	$action = 'bulk-delete';
 
 switch($action) {
@@ -154,10 +154,10 @@ endif; ?>
 <div class="tablenav">
 
 <?php
-$pagenum = absint( $_GET['pagenum'] );
+$pagenum = isset( $_GET['pagenum'] ) ? absint( $_GET['pagenum'] ) : 0;
 if ( empty($pagenum) )
 	$pagenum = 1;
-if( !$catsperpage || $catsperpage < 0 )
+if( ! isset( $catsperpage ) || $catsperpage < 0 )
 	$catsperpage = 20;
 
 $page_links = paginate_links( array(
diff --git a/wp-admin/edit-category-form.php b/wp-admin/edit-category-form.php
index ef11b74a49..d1d8510add 100644
--- a/wp-admin/edit-category-form.php
+++ b/wp-admin/edit-category-form.php
@@ -6,6 +6,12 @@
  * @subpackage Administration
  */
 
+/**
+ * @var object
+ */
+if ( ! isset( $category ) )
+	$category = (object) array();
+
 if ( ! empty($cat_ID) ) {
 	/**
 	 * @var string
@@ -24,6 +30,29 @@ if ( ! empty($cat_ID) ) {
 	$nonce_action = 'add-category';
 	do_action('add_category_form_pre', $category);
 }
+
+/**
+ * @ignore
+ * @since 2.7
+ * @internal Used to prevent errors in page when no category is being edited.
+ *
+ * @param object $category
+ */
+function _fill_empty_category(&$category) {
+	if ( ! isset( $category->name ) )
+		$category->name = '';
+
+	if ( ! isset( $category->slug ) )
+		$category->slug = '';
+
+	if ( ! isset( $category->parent ) )
+		$category->parent = '';
+
+	if ( ! isset( $category->description ) )
+		$category->description = '';
+}
+
+_fill_empty_category($category);
 ?>
 
 <div class="wrap">
diff --git a/wp-admin/edit-form-advanced.php b/wp-admin/edit-form-advanced.php
index 3da40273ad..85ecc3edee 100644
--- a/wp-admin/edit-form-advanced.php
+++ b/wp-admin/edit-form-advanced.php
@@ -369,7 +369,7 @@ function post_password_meta_box($post) {
 	<label for="post_status_private" class="selectit"><input id="post_status_private" name="post_status" type="checkbox" value="private" <?php checked($post->post_status, 'private'); ?> tabindex="4" /> <?php _e('Keep this post private') ?></label>
 </p>
 <h4><?php _e( 'Post Password' ); ?></h4>
-<p><label class="hidden" for="post_password"><?php _e('Password Protect This Post') ?></label><input name="post_password" type="text" size="25" id="post_password" value="<?php echo attribute_escape( $post->post_password ); ?>" /></p>
+<p><label class="hidden" for="post_password"><?php _e('Password Protect This Post') ?></label><input name="post_password" type="text" size="25" id="post_password" value="<?php if ( isset( $post->post_password ) ) echo attribute_escape( $post->post_password ); ?>" /></p>
 <p><?php _e('Setting a password will require people who visit your blog to enter the above password to view this post and its comments.'); ?></p>
 <?php
 }
diff --git a/wp-admin/edit-link-categories.php b/wp-admin/edit-link-categories.php
index f8c4ff1022..d73f9b0efb 100644
--- a/wp-admin/edit-link-categories.php
+++ b/wp-admin/edit-link-categories.php
@@ -95,10 +95,10 @@ endif; ?>
 <div class="tablenav">
 
 <?php
-$pagenum = absint( $_GET['pagenum'] );
+$pagenum = isset( $_GET['pagenum'] ) ? absint( $_GET['pagenum'] ) : 0;
 if ( empty($pagenum) )
 	$pagenum = 1;
-if( !$catsperpage || $catsperpage < 0 )
+if( ! isset( $catsperpage ) || $catsperpage < 0 )
 	$catsperpage = 20;
 
 $page_links = paginate_links( array(
diff --git a/wp-admin/edit-link-category-form.php b/wp-admin/edit-link-category-form.php
index a4b22edce7..5171feb734 100644
--- a/wp-admin/edit-link-category-form.php
+++ b/wp-admin/edit-link-category-form.php
@@ -6,6 +6,12 @@
  * @subpackage Administration
  */
 
+/**
+ * @var object
+ */
+if ( ! isset( $category ) )
+	$category = (object) array();
+
 if ( ! empty($cat_ID) ) {
 	/**
 	 * @var string
@@ -24,6 +30,26 @@ if ( ! empty($cat_ID) ) {
 	$nonce_action = 'add-link-category';
 	do_action('add_link_category_form_pre', $category);
 }
+
+/**
+ * @ignore
+ * @since 2.7
+ * @internal Used to prevent errors in page when no category is being edited.
+ *
+ * @param object $category
+ */
+function _fill_empty_link_category(&$category) {
+	if ( ! isset( $category->name ) )
+		$category->name = '';
+
+	if ( ! isset( $category->slug ) )
+		$category->slug = '';
+
+	if ( ! isset( $category->description ) )
+		$category->description = '';
+}
+
+_fill_empty_link_category($category);
 ?>
 
 <div class="wrap">
diff --git a/wp-admin/edit-tag-form.php b/wp-admin/edit-tag-form.php
index 909c3a7930..c76f1b8309 100644
--- a/wp-admin/edit-tag-form.php
+++ b/wp-admin/edit-tag-form.php
@@ -36,12 +36,12 @@ if ( ! empty($tag_ID) ) {
 	<table class="form-table">
 		<tr class="form-field form-required">
 			<th scope="row" valign="top"><label for="name"><?php _e('Tag name') ?></label></th>
-			<td><input name="name" id="name" type="text" value="<?php echo attribute_escape($tag->name); ?>" size="40" aria-required="true" />
+			<td><input name="name" id="name" type="text" value="<?php if ( isset( $tag->name ) ) echo attribute_escape($tag->name); ?>" size="40" aria-required="true" />
             <p><?php _e('The name is how the tag appears on your site.'); ?></p></td>
 		</tr>
 		<tr class="form-field">
 			<th scope="row" valign="top"><label for="slug"><?php _e('Tag slug') ?></label></th>
-			<td><input name="slug" id="slug" type="text" value="<?php echo attribute_escape(apply_filters('editable_slug', $tag->slug)); ?>" size="40" />
+			<td><input name="slug" id="slug" type="text" value="<?php if ( isset( $tag->slug ) ) echo attribute_escape(apply_filters('editable_slug', $tag->slug)); ?>" size="40" />
             <p><?php _e('The &#8220;slug&#8221; is the URL-friendly version of the name. It is usually all lowercase and contains only letters, numbers, and hyphens.'); ?></p></td>
 		</tr>
 	</table>
diff --git a/wp-admin/edit-tags.php b/wp-admin/edit-tags.php
index a5c2b309b2..1c56384c70 100644
--- a/wp-admin/edit-tags.php
+++ b/wp-admin/edit-tags.php
@@ -13,7 +13,7 @@ $title = __('Tags');
 
 wp_reset_vars(array('action', 'tag'));
 
-if ( $_GET['action'] == 'delete' && isset($_GET['delete_tags']) )
+if ( isset( $_GET['action'] ) && $_GET['action'] == 'delete' && isset($_GET['delete_tags']) )
 	$action = 'bulk-delete';
 
 switch($action) {
@@ -161,10 +161,10 @@ endif; ?>
 <div class="tablenav">
 
 <?php
-$pagenum = absint( $_GET['pagenum'] );
+$pagenum = isset( $_GET['pagenum'] ) ? absint( $_GET['pagenum'] ) : 0;
 if ( empty($pagenum) )
 	$pagenum = 1;
-if( !$tagsperpage || $tagsperpage < 0 )
+if( ! isset( $tagsperpage ) || $tagsperpage < 0 )
 	$tagsperpage = 20;
 
 $page_links = paginate_links( array(
@@ -201,7 +201,7 @@ if ( $page_links )
 	<tbody id="the-list" class="list:tag">
 <?php
 
-$searchterms = trim( $_GET['s'] );
+$searchterms = isset( $_GET['s'] ) ? trim( $_GET['s'] ) : '';
 
 $count = tag_rows( $pagenum, $tagsperpage, $searchterms );
 ?>
diff --git a/wp-admin/includes/user.php b/wp-admin/includes/user.php
index 8a89ac8483..4400008d55 100644
--- a/wp-admin/includes/user.php
+++ b/wp-admin/includes/user.php
@@ -230,10 +230,10 @@ function get_user_to_edit( $user_id ) {
 	$user->last_name    = attribute_escape($user->last_name);
 	$user->display_name = attribute_escape($user->display_name);
 	$user->nickname     = attribute_escape($user->nickname);
-	$user->aim          = attribute_escape($user->aim);
-	$user->yim          = attribute_escape($user->yim);
-	$user->jabber       = attribute_escape($user->jabber);
-	$user->description  =  wp_specialchars($user->description);
+	$user->aim          = isset( $user->aim ) && !empty( $user->aim ) ? attribute_escape($user->aim) : '';
+	$user->yim          = isset( $user->yim ) && !empty( $user->yim ) ? attribute_escape($user->yim) : '';
+	$user->jabber       = isset( $user->jabber ) && !empty( $user->jabber ) ? attribute_escape($user->jabber) : '';
+	$user->description  = isset( $user->description ) && !empty( $user->description ) ? wp_specialchars($user->description) : '';
 
 	return $user;
 }
diff --git a/wp-admin/includes/widgets.php b/wp-admin/includes/widgets.php
index f7dc77f97c..2797ae5c7c 100644
--- a/wp-admin/includes/widgets.php
+++ b/wp-admin/includes/widgets.php
@@ -59,9 +59,11 @@ function wp_list_widgets( $show = 'all', $_search = false ) {
 			if ( ( 'unused' == $show && $sidebar ) || ( 'used' == $show && !$sidebar ) )
 				continue;
 
+			if ( ! isset( $widget['params'][0] ) )
+				$widget['params'][0] = array();
 			ob_start();
-				$args = wp_list_widget_controls_dynamic_sidebar( array( 0 => array( 'widget_id' => $widget['id'], 'widget_name' => $widget['name'], '_display' => 'template', '_show' => $show ), 1 => $widget['params'][0] ) );
-				$sidebar_args = call_user_func_array( 'wp_widget_control', $args );
+			$args = wp_list_widget_controls_dynamic_sidebar( array( 0 => array( 'widget_id' => $widget['id'], 'widget_name' => $widget['name'], '_display' => 'template', '_show' => $show ), 1 => $widget['params'][0] ) );
+			$sidebar_args = call_user_func_array( 'wp_widget_control', $args );
 			$widget_control_template = ob_get_contents();
 			ob_end_clean();
 
@@ -230,6 +232,13 @@ function wp_widget_control( $sidebar_args ) {
 	$edit = -1 <  $edit_widget && is_numeric($key) && $edit_widget === $key; // (bool) are we currently editing this widget
 
 	$id_format = $widget['id'];
+
+	if ( ! isset( $sidebar_args['_show'] ) )
+		$sidebar_args['_show'] = '';
+
+	if ( ! isset( $sidebar_args['_display'] ) )
+		$sidebar_args['_display'] = '';
+
 	// We aren't showing a widget control, we're outputing a template for a mult-widget control
 	if ( 'all' == $sidebar_args['_show'] && 'template' == $sidebar_args['_display'] && isset($control['params'][0]['number']) ) {
 		// number == -1 implies a template where id numbers are replaced by a generic '%i%'
diff --git a/wp-admin/menu.php b/wp-admin/menu.php
index 2d77713131..20ee20e397 100644
--- a/wp-admin/menu.php
+++ b/wp-admin/menu.php
@@ -124,7 +124,9 @@ foreach ( $menu as $id => $data ) {
 			unset($submenu[$old_parent][$index]);
 		}
 		unset($submenu[$old_parent]);
-		$_wp_submenu_nopriv[$new_parent] = $_wp_submenu_nopriv[$old_parent];
+
+		if ( isset($_wp_submenu_nopriv[$old_parent]) )
+			$_wp_submenu_nopriv[$new_parent] = $_wp_submenu_nopriv[$old_parent];
 	}
 }
 
diff --git a/wp-admin/upload.php b/wp-admin/upload.php
index 984ff24162..72ada70f8a 100644
--- a/wp-admin/upload.php
+++ b/wp-admin/upload.php
@@ -177,7 +177,7 @@ if ( isset($_GET['detached']) ) {
 		$h2_noun = $post_mime_type_label;
 	// Use $_GET instead of is_ since they can override each other
 	$h2_author = '';
-	$_GET['author'] = (int) $_GET['author'];
+	$_GET['author'] = isset( $_GET['author'] ) ? (int) $_GET['author'] : 0;
 	if ( $_GET['author'] != 0 ) {
 		if ( $_GET['author'] == '-' . $user_ID ) { // author exclusion
 			$h2_author = ' ' . __('by other authors');
diff --git a/wp-admin/user-edit.php b/wp-admin/user-edit.php
index ad5e2731a7..260555fb94 100644
--- a/wp-admin/user-edit.php
+++ b/wp-admin/user-edit.php
@@ -157,7 +157,7 @@ include ('admin-header.php');
 	<?php endif; ?>
 </div>
 <?php endif; ?>
-<?php if ( is_wp_error( $errors ) ) : ?>
+<?php if ( isset( $errors ) && is_wp_error( $errors ) ) : ?>
 <div class="error">
 	<ul>
 	<?php
diff --git a/wp-includes/feed.php b/wp-includes/feed.php
index a8ae704432..535a80de8d 100644
--- a/wp-includes/feed.php
+++ b/wp-includes/feed.php
@@ -494,7 +494,7 @@ function prep_atom_text_construct($data) {
  */
 function self_link() {
 	echo 'http'
-		. ( $_SERVER['https'] == 'on' ? 's' : '' ) . '://'
+		. ( (isset($_SERVER['https']) && $_SERVER['https'] == 'on') ? 's' : '' ) . '://'
 		. $_SERVER['HTTP_HOST']
 		. wp_specialchars(stripslashes($_SERVER['REQUEST_URI']), 1);
 }
diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index 76bcaf93f3..24ea68f2a7 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -802,10 +802,8 @@ function maybe_serialize( $data ) {
  */
 function make_url_footnote( $content ) {
 	preg_match_all( '/<a(.+?)href=\"(.+?)\"(.*?)>(.+?)<\/a>/', $content, $matches );
-	$j = 0;
+	$links_summary = "\n";
 	for ( $i=0; $i<count($matches[0]); $i++ ) {
-		$links_summary = ( !$j ) ? "\n" : $links_summary;
-		$j++;
 		$link_match = $matches[0][$i];
 		$link_number = '['.($i+1).']';
 		$link_url = $matches[2][$i];
diff --git a/wp-includes/taxonomy.php b/wp-includes/taxonomy.php
index c0267fd4b8..98d48fca0c 100644
--- a/wp-includes/taxonomy.php
+++ b/wp-includes/taxonomy.php
@@ -715,7 +715,8 @@ function &get_terms($taxonomies, $args = '') {
 	if ( empty($terms) ) {
 		$cache[ $key ] = array();
 		wp_cache_set( 'get_terms', $cache, 'terms' );
-		return apply_filters('get_terms', array(), $taxonomies, $args);
+		$terms = apply_filters('get_terms', array(), $taxonomies, $args);
+		return $terms;
 	}
 
 	if ( $child_of || $hierarchical ) {