Yes, saw it only in VC6. VC6 does not automatically allow editing readonly files. If the file is not under source control, VC6 will just ignore what you type. But the file is added to the history anyway.
Usually using Alt+O (mapped to "Open file in workspace" for me), also by right-clicking on a #include and choosing "Open <...>" Just tried to open a readonly file through the "FileView" tab, external dependencies. Same thing.