Index: wp-admin/link-add.php
===================================================================
--- wp-admin/link-add.php	(revision 6574)
+++ wp-admin/link-add.php	(working copy)
@@ -16,7 +16,7 @@
 require('admin-header.php');
 ?>
 
-<?php if ($_GET['added'] && '' != $_POST['link_name']) : ?>
+<?php if ( ! empty($_GET['added']) && '' != $_POST['link_name']) : ?>
 <div id="message" class="updated fade"><p><?php _e('Link added.'); ?></p></div>
 <?php endif; ?>
 
Index: wp-admin/edit.php
===================================================================
--- wp-admin/edit.php	(revision 6574)
+++ wp-admin/edit.php	(working copy)
@@ -4,7 +4,7 @@
 $title = __('Posts');
 $parent_file = 'edit.php';
 wp_enqueue_script( 'admin-posts' );
-if ( 1 == $_GET['c'] )
+if ( isset($_GET['c']) && 1 == $_GET['c'] )
 	wp_enqueue_script( 'admin-comments' );
 require_once('admin-header.php');
 
@@ -25,13 +25,13 @@
 if ( is_single() ) {
 	printf(__('Comments on %s'), apply_filters( "the_title", $post->post_title));
 } else {
-	if ( $post_listing_pageable && !is_archive() && !is_search() )
+	if ( isset($post_listing_pageable) && $post_listing_pageable && !is_archive() && !is_search() )
 		$h2_noun = is_paged() ? sprintf(__( 'Previous %s' ), $post_status_label) : sprintf(__('Latest %s'), $post_status_label);
 	else
-		$h2_noun = $post_status_label;
+		$h2_noun = ( isset($post_status_label) ) ? $post_status_label : null;
 	// 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');
@@ -105,7 +105,7 @@
 <?php } ?>
 
 	<fieldset><legend><?php _e('Category&hellip;') ?></legend>
-		<?php wp_dropdown_categories('show_option_all='.__('All').'&hide_empty=1&hierarchical=1&show_count=1&selected='.$cat);?>
+		<?php wp_dropdown_categories('show_option_all='.__('All').'&hide_empty=1&hierarchical=1&show_count=1&selected='.((isset($cat))?$cat:0));?>
 	</fieldset>
 	<input type="submit" id="post-query-submit" value="<?php _e('Filter &#187;'); ?>" class="button" />
 </form>