Here's a fun one. A site collection was created on a development portal to test database restores. When the database backup was restored to the portal, SharePoint lost all access to the site collection. The site collection was still listed, but no information returned about the database, and of course no access to the portal URL. As SharePoint couldn't access the database, it was deleted from SQL. Easy enough, but how do we get rid of the site collection?
I found a good post at TechNet, which basically says to do the following:
Also, don't forget to remove the content database from SharePoint Central Administration!