Story related so you can’t miss it.
SPOILERS AHEAD
You’ll get this one at test chamber 21. You’ll be faced with what looks like a fairly easy test. A light bridge which you’ll use to get up to a button and catch a cube from falling into the water/acid stuff. Just go up and press the small button and a cube will drop. As you go to retrieve your cube the lights will go out (including your light bridge) and you’ll fall to the ground. GLaDOS will be surprised and Wheatley will open up a panel in the wall. Go over to him and when the light bridge activates again walk across it to the catwalks behind and (listen to Wheatley if you want for a bit) start walking down the catwalk. The achievement should pop after a few steps.