The game can be completed without incinerating the companion cube, or the cube can be rescued from the incinerator (without the use of console commands or exploiting a bug to get a crate past an emancipation grid).
It is possible to assume the party escort submission position by pressing down WASD on the PC version or the direction arrows on the Console versions.
The G-Man can be seen walking around the Testing Area.
The cake is a lie.
As seen at the end of the game, the cake actually exists.