After installing windows 8 RTM and starting to work on my project I found there is a problem in my Usercontrols that was built in a separate project it always show and exception “Xaml Parsing Failed”
this error really got me really hard I traced down the problem I found out that the problem is because of a the . in my Assembly name I was Generating from that project “was working great in early release of Visual Studio”
anyway it was declared as error by Microsoft and they are doing to release a Hotfix for it soon. as a Quick correction you can remove the DOT from your assembly name.
Check the below Article on technet for more details.