SEO advice: Don't move around pages

Preferably don't move pages around, but if you do, make sure the old address will redirect to the new address with a 301 Moved Permanently status code.

Case in point:

I just searched for "paypal donation button" the first link was an article with the title "Guide to Creating a PayPal Donate Button for your Website.". on PayPal so I clicked on it. I got a 404 Not Found error with the text "Oops. That page doesn't exist."

PayPal might be able to afford such stupidity. You don't.