Set error_reporting(-1) in wp-load.php and wp-includes/load.php to catch all warnings. Wordpress has fixed all of them in the current version. Previously we saw warnings like this:

Strict Standards: Declaration of Walker_Page::start_lvl() should be compatible with that of Walker::start_lvl() in wp-includes/classes.php on line 1229
Strict Standards: Declaration of Walker_Page::end_lvl() should be compatible with that of Walker::end_lvl() in wp-includes/classes.php on line 1229
Strict Standards: Declaration of Walker_Page::start_el() should be compatible with that of Walker::start_el() in wp-includes/classes.php on line 1229
Strict Standards: Declaration of Walker_Page::end_el() should be compatible with that of Walker::end_el() in wp-includes/classes.php on line 1229
Strict Standards: Declaration of Walker_PageDropdown::start_el() should be compatible with that of Walker::start_el() in wp-includes/classes.php on line 1274
Strict Standards: Declaration of Walker_Category::start_lvl() should be compatible with that of Walker::start_lvl() in wp-includes/classes.php on line 1421
Strict Standards: Declaration of Walker_Category::end_lvl() should be compatible with that of Walker::end_lvl() in wp-includes/classes.php on line 1421
Strict Standards: Declaration of Walker_Category::start_el() should be compatible with that of Walker::start_el() in wp-includes/classes.php on line 1421
Strict Standards: Declaration of Walker_Category::end_el() should be compatible with that of Walker::end_el() in wp-includes/classes.php on line 1421
Strict Standards: Declaration of Walker_CategoryDropdown::start_el() should be compatible with that of Walker::start_el() in wp-includes/classes.php on line 1472
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/classes.php on line 1671 Strict Standards: Redefining already defined constructor for class wpdb in wp-includes/wp-db.php on line 481
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/cache.php on line 103
Strict Standards: Redefining already defined constructor for class WP_Object_Cache in wp-includes/cache.php on line 482
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/l10n.php on line 453
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/query.php on line 61
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/theme.php on line 1335
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/theme.php on line 1426
Strict Standards: Declaration of Walker_Comment::start_lvl() should be compatible with that of Walker::start_lvl() in wp-includes/comment-template.php on line 1349
Strict Standards: Declaration of Walker_Comment::end_lvl() should be compatible with that of Walker::end_lvl() in wp-includes/comment-template.php on line 1349
Strict Standards: Declaration of Walker_Comment::start_el() should be compatible with that of Walker::start_el() in wp-includes/comment-template.php on line 1349
Strict Standards: Declaration of Walker_Comment::end_el() should be compatible with that of Walker::end_el() in wp-includes/comment-template.php on line 1349
Strict Standards: Redefining already defined constructor for class WP_Dependencies in wp-includes/class.wp-dependencies.php on line 33
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/taxonomy.php on line 723
Strict Standards: Redefining already defined constructor for class WP_Embed in wp-includes/media.php on line 961
Strict Standards: Redefining already defined constructor for class WP_Widget in wp-includes/widgets.php on line 93
Deprecated: Assigning the return value of new by reference is deprecated in wp-includes/widgets.php on line 324
Annoying useless code in class.wp-dependencies.php
<?php
class WP_Dependencies {
        function 
WP_Dependencies() {
                
$args func_get_args();
                
call_user_func_array( array(&$this'__construct'), $args );
        }

        function 
__construct() {}
        ...
Examples of warning fixes
<?php
--- ./cache.php	2010-03-05 09:31:16.573829617 -0800
+++ ../../wp2/wp-includes/cache.php	2010-03-05 16:11:39.502645987 -0800
@@ -100,7 +100,7 @@
-	$GLOBALS['wp_object_cache'] =& new WP_Object_Cache();
+	$GLOBALS['wp_object_cache'] = new WP_Object_Cache();

--- ./classes.php	2010-03-05 09:31:16.603847118 -0800
+++ ../../wp2/wp-includes/classes.php	2010-03-05 13:30:32.957323183 -0800
@@ -779,7 +779,7 @@
-	function start_lvl(&$output) {}
+#	function start_lvl(&$output) {}
 
 	/**
 	 * Ends the list of after the elements are added.
@@ -793,7 +793,7 @@
-	function end_lvl(&$output)   {}
+#	function end_lvl(&$output)   {}
Getting rid of PHP-4 style new references and stub methods and constructors that serve no purpose.