The old Turrets were meant to have voices, but they had to be scrapped due to technical difficulties. The unused voices can be found in the game's files.
After completing the first level "Funnel Over Goo", in between the two chambers is a companion cube hidden underneath the ground. Attempting to pick up the cube will cause it to disintegrate and give the player the "Forever Alone" achievement.
In the first few minutes of the game, a recording can be heard of a man named Chris being fired by Cave Johnson for misspelling 'station' in one of Cave's briefings. This is an inside joke, as Chris is a member of the development team who was routinely "fired" by the other team members whenever he made a mistake.