In less than a week Microsoft will formally end support for versions 4.0, 4.5, and 4.5.1 of the .NET Framework. When upgrading to .NET 4.5.2, ASP.NET developers may see a compatibility issue.
http://www.infoq.com/news/2016/01/Support-Ending
http://www.infoq.com/news/2016/01/Support-Ending