It's nice to have a complete project at your disposal to see how an application works when it's modified.
That's in an ideal situation. Now, say you have ftp access to a web site root with all the source files, you're unfamiliar with the site structure, you have no direct access to the database (so you can only guess the structure...) and the site just happens to be live.
How do you go about adding some features in a context like this?
P.S: Title is a reference to so-called keyhole surgery.