I've run into this problem a couple of times with CS. I programmatically create blogs as required to display content. At the same time I am generating a unique Application Key for the Blog. The problem is that CS (2008.5 and previous atleast) do a URL rewrite and blog lookup based on that Application Key, but it is case sensitive. Sometimes CS will ignore the case and find the Blog anyway, but sometimes it will treat them as different and report a "Weblog does not exist" exception. I've gotten around this by making sure all my links to blogs are converted to lower case first.