This happened to me in a client's WSS3 environment, but it seems to apply to 2010 as well. I had a site template created in one environment, but upon moving it to another, the Create Site operation resulted in a neat little error, complete with stack...
If one day, for no apparent reason, you notice that your default SharePoint content types have disappeared, you may have made the simple, unwitting mistake of reusing a GUID in a custom content type. This can cause a very confused SP server to hide all...
