Development Gag: In the Video Games ending, jumping down the elevator shaft after completing the Portal map will bring the player back to the office, as it appeared in the original mod.
Doing It for the Art: Kevan Brighting's voice work as The Narrator is not only found in both the original mod and the HD Remix release, but he has provided his soothing tones for a little-watched seven-minute YouTube video called The Aperture Parable, in which he describes how the nature of The Stanley Parable is similar to that of Portal. He also role-plays as The Narrator on his Twitteraccount, continuing the never-ending adventures of Stanley.
Hey, It's That Guy!: Chilled Chaos, a relatively well-known Let's Player (who also played through the original Stanley Parable mod), helped out with one of the trailers (this one) by essentially doing his usual shtick while playing through some of the game.
What Could Have Been: The Museum ending, aside from showing off a lot of the game's assets, demonstrates several unused and unimplemented ideas, including further Ask The Narrator trailers (in the vein of the Raphael trailer) and a battleground ending.
A planned ending, where the game turns into a sci-fi First-Person Shooter game with its own narrator who ends up in conflict with the Narrator, was never implemented, but shows up in the Museum.