well, most of them wont be noticed by anyone.
since upgrading to WordPress 2.x, 404 errors havent been working correctly. WP2 has the index.php handle all 404 requests. which is fine if you use a WP theme, but i dont. the default WP theme's setup is designed to handle any 404 request thrown at it. i had to work some of that code into my site to make things work. i didnt know of any other way to make 404s work again. the WordPress.org website has a ton of posts in their forum where people cant get 404s to work. none of the "solutions" posted there helped me.
besides getting 404s working again, i also changed it so every page in the archive will be titled. when my site was spidered, every page in the archive was titled "Xenomorph dot Net", which made the site map i had almost useless.