When using Visual Basic 6.0, if you tried to edit a read-only file (a checked-in file under source control, or a “by hand” read-only file), you got an error messagebox and that was all. However, when using Visual Studio .NET, if you try to edit a read-only file, a lot of things can happen depending on several dialog prompts and configurable settings. Even things that IMHO should not be allowed by Visual Studio, such as allowing editing a read-only file and deferring the problem of what to do with the changes when trying to save it. When writing an add-in that tries to modify a file, you should take all them into account:

HOWTO: Understanding Visual Studio behavior when an add-in tries to edit a read-only file
http://www.mztools.com/articles/2009/MZ2009009.aspx