Why does machine.config contain invalid Xml content after installing .Net 3.5 patches?

For quite a few times, I heard customers would hit this issue after installing .Net 3.5 patches or repair 3.5 on Windows Vista or Windows 2008 Server. Basically the machine.config file contains some invalid Xml content and applications using configuration do not work, especially for IIS-hosted applications. The main problem is that the WCF 3.5 installer (WFServiceReg.exe) did not handle the