Everything that has an end, has a beginning...
You are not logged in.
The part where you recreated the behind the scenes area of the official Portal game, but now you need to use the two cubes to climb into the vent where the turrets are littered in the room that has the maintenance elevator that brings you back into the game.
To get stuck you jump on the two cubes to get into the vent, jump in the vent and put a portal in the room with the turrets. With a portal now in that room, fall back into the area you just came from, throw both cubes through the portal, and then shoot the exit portal color in the room you are in now (so now you have both portals in the same room as you). At this point, you can't get into the room with the turrets unless you reload the game.
If you didn't follow, start watching at ~2:51 on the walkthrough which starts in the room you can get stuck in. Hopefully I made this clear enough.
Offline
Yes, it can happen.
But still, you could go back and use the two other cubes located in the 15th test chamber.
Then, if you decided to throw these two supplementary cubes too, there's nothing we can do...
I mean, if you throw away the cubes two times while knowing you would end up stuck, then it's mostly the player's fault!
I know what you mean, "if it's possible, then it's a level-design error" and you're partially right, but still, that sounds unlikely to happen.
Offline