hopefully SOMEONE will read this and this post is not all in vain... but....
i suppose a reboot would fix that. however. when this error occurs (And it happens to me a lot, especially because ive renamed the whole projects namespace several times.) this is the quick fix:
lets say the name of your project is "MyPj"
you MUST have visual studio open (assuming this is what you use) and your project fully loaded to the extend that you can "Rebuild MyPj" at any time during this..
then, open the folder containing the project and delete all project files pertaining to this project: specifically: MyPj.sln, MyPj.suo, MyPj.webinfo, MyPj.project, etc... just delete everything that starts with "MyPj"
after this has been done, go back to visual studio and click "Rebuild MyPj"
two or three MyPj.whatever files will be created and placed back into the folder your project is located in. this error should now be 100% gone (at least for this rebuild)
hope this helps