I have seen a few cases of this so I wanted to let everyone know. If you install the FrontPage Server Extensions (FPSE) on a web server, it will install some configuration files in a directory under your web site called _vti_cnf. This folder contains a bunch of files that FPSE use to update files and perform the various tasks that it needs to do.
The problem comes if that _vti_cnf folder is not marked hidden. If something keeps that from happening, you can get a few different errors because of it. The reason is that the aspnet_compiler.exe will try to compile these files since the folder isn't hidden. There is a web.config file in there that will cause an error like:
Invalid token 'Text' at root level of document. XML document must contain a root level element. Data at the root level is invalid. Line 1, position 1.
As long as this folder is hidden, the compiler will just skip over it and everything will be fine.
So the main thing to remember is that if you install FPSE and suddenly start getting errors, checking if that folder is marked hidden is a great place to start.