Something bad happened. I’d like to tell you more about it, but only if you can remember to start Visual Studio with an additional command-line parameter, and then recreate the conditions that caused it.
Even a stack trace would have been more useful than the general “Oh, crud” dialog.
Or, just hit the OK button.
(I’ve noticed this error happening with some regularity now that I’ve been exploring ASP.NET MVC 3 with Razor syntax more. Have you seen this error?)