I was getting the error “Package ‘Visual Studio Explorers and Designers Package’ has failed to load properly” when working with add-ins in VS 2008 since a couple of days ago, along with “COM object that has been separated from its underlying RCW cannot be used” some other times. While I had seen the second error working with add-ins, I had never seen the first one. Browsing the MSDN Visual Studio Extensibility forum today I discovered that I was not the only one, and that the culprit is the Microsoft Source Analysis for C# tool that I installed a couple of days ago, when problems started 🙂