You might have seen this before: You open a project in Visual Studio, and almost every possible keyword is underlined red. Building works just fine, but the editor and Intellisense just freak out. In my case, working in a Windows Phone project, I couldn’t select the target (Device, Emulator 512, etc) in the combo box next to the “Run” button in the taskbar. It only showed “Start”.
Having just installed Xamarin, I first thought that it was the culprit. Or maybe Resharper. Anyway whatever the cause was (and I am still not 100% sure what actually triggered this behavior), recovering from it is actually quite easy: