Portal 1 and 2 have amazing in-game developer commentary modes. Would highly recommend a replay with the commentary enabled to anyone interested in game design at all.
Can't watch with sound at the moment to confirm it's a good video, but this comes up in youtube
I wish more games had dev commentary like this. It's neat to peek into the minds of the developers, and can give good informal lessons on designing games and levels.
I don't know where the parent got their info, but I learned this through the original Portal's in-game "developer commentary" system. If you turn on the dev commentary in Portal they talk about this fact during that level.
Can't watch with sound at the moment to confirm it's a good video, but this comes up in youtube
https://www.youtube.com/watch?v=x0uVg9nuaS0