Portal Chamber 09, what the heck?
That's the impossible one, right? Well, some one else try this.
Go and get the cube on the other side of the fizzler, with the button. Now lean the cube against the door so it will hold it open when the button get pressed. (I find it easiest to jump on top and walk off of it backwards and catch it before it rights itself, then move it into position. Do whatever you want.)
Go through and wait for the cube to get unstuck (It'll either come in with you or go out and be in the button room.) If it comes with you, place a blue portal on a wall, and take the cube through. Now take it cube first through the orange portal. Voila, missing cube.
Fixed, my bad.
Go and get the cube on the other side of the fizzler, with the button. Now lean the cube against the door so it will hold it open when the button get pressed. (I find it easiest to jump on top and walk off of it backwards and catch it before it rights itself, then move it into position. Do whatever you want.)
Go through and wait for the cube to get unstuck (It'll either come in with you or go out and be in the button room.) If it comes with you, place a blue portal on a wall, and take the cube through. Now take it cube first through the orange portal. Voila, missing cube.
I was thinking of Chamber 05 but it's actually Chamber 09. How you could mess it up?! 
It's because the map name is testchmb_a_05.
If so, then there's a gap between Chamber 11 (16) and 13 (17). Check out.
No, no no, it's because some maps include more than 1 chamber.
I didn't mean that, I meant there's an actual gap. There's no map called "testchmb_a_12".
It also happens if you pick up the cube and walk backwards through your blue portal. It makes sense now. It's incase you do exactly what I just told you to, but don't go in the room. Cube is destroyed, since you can't get to it.
It's fun to get it stuck, then open the door. The cube FLIES out. would be cool to see a puzzle where the only way to get a cube across a big gap is that way.
In a recent stream, valve news network asked valve if there was any cut chambers. jeep barnett came with an answer and said that there was a cut chamber with a cage of energy pellets flying around and a button inside. the goal was to get the cube on the button, the energy balls would fly out of the portal. that was testchmb_a_12.