If you open a file that's read only, you'll be able to make changes in the editor, but of course won't be able to save. Currently, you just get an "Access Denied" exception.
The editor should at lease provide some feedback that this is a read-only document and disable the "Save" command. ("Save as" is still okay.)
This may be a task rather than a bug, but I can't submit tasks, so I'll call it a bug. :)