You know there are a lot of people out there that think that they know how to build a good user friendly and SEO friendly website. Unfortunatly for most people there aren't really that many. One of the biggest things that I keep seeing are custom 404 error pages. A 404 Error is a "Page Not Found" issue. You can create a page that will be displayed to the user and/or search engine spider if they come accross a bad link or a link to a page that has been removed from the site.
This is such a bad idea. For the user it serves no purpose except to make your site look like it has a page for anything. For the spiders they keep getting the same page over and over if you have a lot of bad links. If your site has been up for a while and you add, update and remove pages you will not be doing yourself any favors by doing this. The spiders want to save time and only crawl pages that are relivant. If you keep returning the same page that is not really there that page will not be updated in the search engines as quickly. But when the spiders figure it out it will most likely hurt your rankings all over.
So what should you do. Well you can always return the 404 error, create a page that does return a 404 error but looks better to you or return a 301 redirect.
A 301 redirect, redirects the user and spider to another page. The spider reads it as a permant change, the user just sees the page you are redirecting them too. This solves the issue and actually allows you to control traffic.
If you have a product page and that product has been replaced with a new one. You created a new page and pulled the old one down. You would get on your server and create a 301 redirect to the new page. So all of the old links that are on other sites now redirect to the new page. When a spider hits one of those links they are redirected as well.
Don't confuse a 301 redirect with a 302. 301 is like having your mail forwarded because you moved, a 302 is like having a post office box that is then forwarded to your house. People have used 302's for spam and there for the search engines look on them in a negative light.
In any case using custom 404 error pages will hurt your site unless they actually return the 404 error to the browser, which 99% do not. So becareful with those custom error pages.
