Got this really strange error in Visual Studio 2012 today:
"ASP.NET 4.5.256 has not been registered on the Web server. You need to manually configure your Web server for ASP.NET 4.5.256 in order for your site to run correctly."
Except… the whole message makes no sense considering the side by side installation of ASP.NET 4/4.5. The message would popup in my solution while loading ASP.NET projects, but after clicking the OK button it did not appear to have any residual effects on any of those projects files.
Apparently there is a hotfix for this little known bug that you can download from the Microsoft Support. Close Visual Studio before running the hotfix and you probably can avoid a PC restart. Took me a while to find this issue so here it is for future reference.