Drop Box Portal 2 - Place a cube on a button without touching the cube
1Chapter 2, Test Chamber 6 (same room as Final Transmission). After you hit the button for the trash, shoot a portal at the first slanted platform you enter, and the look at the button where the cube is meant to go. Above it is a white wall. Shoot your second portal there. Press the button for the cube to drop and you'll get the Trophy / Achievement earned.
Alternate Strategy from Edd Ryves: You can also get this in Chapter 3 in the area where you first encounter the turrets. The last part of the test area has 3 cubes in it. Use 2 cubes to defeat the turrets in the room but do not touch one of the 3. when you have got the room cleared stand on the button and shoot a portal directly up, then return the the unmoved cube and shoot a portal directly underneath it and presto!