What do the red exclamation mark icons in front of references mean in VS2010?

Question

In Visual Studio 2010 project property pages, if I select Common Properties and Framework and References for a C++ project, I can see all the references from a project.

They usually have this icon:
Sometimes, though, some references appear as this:

I tried to google it or find any documentation about it in the msdn documentation about references, but cannot find anything related to this. Does someone know about this? Thank you very much in advance!

Answer 1

This happens also if you had VS2010 SP1 and then install .net 4.5. They changed the handling of this in a way that causes problem in VS2010. (.net 4.5 is installed with VS2012)