You don't have to be worried, it is just a Visual Studio problem and, if I remember correctly, it only appears when one your libraries depends on the code you recompile but does not have a reference set to it.

Example:
You have lib A and lib B. Lib B depends upon lib A but you do not have a dependency reference set in the project settings for lib B. Now when you recompile lib A there won't be a new lib B and thus the old lib B tries to access the new version of lib A. This can work, but usually does not and therefore leads to runtime errors.

I might be totally wrong, though, as it is a long time ago that it happened to me and then read about why it happens.