I have recently upgraded to VS 2012, I had to as I needed to start using the .net 4.5 but that is besides the point. My problem is the following:
I have a Resource
I also have this problem time to time in VS2010. Sometimes the problem will solve if I make a "Clean Solution" and a "Rebuild Solution". If that do not work I usually restart VS2010.
I have also renamed the Style x:Key to something else and the problem was gone. But I dont find this solution to be perfect...
Reference to this question regarding the Workaround in my code sample: Trouble referencing a Resource Dictionary that contains a Merged Dictionary