Good Listener Achievement
Take GLaDOS' escape advice
This can be gained in Chapter 5's escape.
While escaping GlaDOS with Wheatley, you'll come to a part where she'll reveal the 'last test chamber' and a light bridge will form. Instead of going right and following Wheatly, take the left route and into the 'last chamber'.
Once inside you'll be confronted by a nice surprise and the achievement will unlock.