Those previous two posts were meant to help people unfamiliar with Office XML get a taste of how you would go about building your own spreadsheets. The power of doing this with XML is that you can read and generate these files anywhere you want. You don't need Office on the machines you are using to work with the files. This is especially important when dealing with large numbers of documents or as part of a web service, as the Office applications are not supported running in an unattended mode on a server. XML lets you do it in a supported, and very efficient way.
As you start to experiment with XML in the Office applications, you will no doubt create XML files that can't be opened. By default there aren't great error messages reported, because we don't want to confuse the average end user. If the file can't be opened, then it can't be opened, there isn't much that the average user can do to fix it. If you are a developer or solution builder though and you actually generated that file, then it would be nice to get some more information on the problem. There are a number of options you have to make debugging your files easier. Here are the steps you should take when you have trouble opening a file: