* TheCastShowoff: Ellen [=McLain=] is an operatic singer, which means she sings one hell of a credits song.
* CreatorBacklash: ''Portal's'' writer, Erik Wolpaw, grew sick of the non-stop cake jokes after the first game. In [[http://www.escapistmagazine.com/news/view/101476-Portal-2-Writer-Enough-With-the-Cake-Jokes-Already this interview]], he claimed there wouldn't be any references to it in the sequel -- only to include things like [[https://www.youtube.com/watch?v=UgLgm_TN1gE one of Doug Rattmann's drawings depicting the cake (at 1:07 in the video)]], and the binary code in Part 1 of the comic translating to "the cake is a lie." In the end, Wolpaw [[http://www.telegraph.co.uk/technology/video-games/8359173/Portal-2-developer-interview-Chet-Falisek-and-Erik-Wolpaw.html just couldn't resist]].
* DVDCommentary: Both games have a "Developer Commentary" option. Here, there are speech bubbles scattered around the maps, and pressing the use key on them gives a fun fact about development.
* {{Defictionalization}}: [[http://www.thinkgeek.com/interests/portal/ed41/?cpg=tw#tabs The Portal Gun ]] Though it obviously can’t create real portals.
* PropRecycling:
** The "High-Energy Pellet" is the same sprite and sound effects as [[VideoGame/HalfLife2 the Combine's energy orbs]]. Its effect on living beings is pretty much the same as well, although Gordon's suit seems to block its damage.
** Chell's leg-springs were part of an enemy cut from ''VideoGame/HalfLife2'', the [[http://combineoverwiki.net/wiki/Combine_Assassin Combine Assassin]].
** The sentry turrets have largely the same behavior as ''Half-Life 2''[='s=] Combine turrets. They also use the same sound effects [[https://www.youtube.com/watch?v=ujYcnO0sIks in the trailer]] [[spoiler:and during ''Portal 2'''s ending]].
** Chell actually vocally reacts to impact damage from cubes and other such things. The sound bites used for this are from female [=NPCs=] who take damage in ''Half-Life 2.'' In ''Portal 2'', though, this doesn't happen.
* SerendipityWritesThePlot:
** According to ''Portal 2 - The Final Hours'', the reason the games take place in the ''Half-Life'' universe was to justify the recycled assets from ''Half-Life 2'', which were a lot more pronounced in early development due to the game's small development team.
** Originally, Cave Johnson's personal assistant was going to be YesMan named Doug, but the developers realized that they'd have to find and pay an actor for what amounted to five lines. To save time and money, they changed Doug to a woman named Caroline and asked Ellen [=McLain=], who was already voicing [=GLaDOS=], to read the part instead. But then the writers got to wondering--what if there was a ''reason'' that Caroline and [=GLaDOS=] had the same voice?...
* ThrowItIn: [[http://www.gamingangels.com/2011/03/pax-east-2011-portal-2-solo-and-an-interview-with-erik-wolpaw/ In this interview]], WordOfGod reveals that the blood splatters weren't meant to be in the game at all. However, they weren't even noticed until too late in development, and were so ingrained into the engine that, by that point, it would have been better to just leave them in the game. Valve went out of their way to remove the blood splatters for the sequel, however, hence its lower rating compared to the original.
* WhatCouldHaveBeen:
** In earlier concepts for a sequel to ''Portal'', Cave Johnson was positioned to play a significantly larger role, including [[spoiler:being subject to BrainUploading (which, in the final release, he ''wanted'', but he likely died before it could happen), and undergoing an existential crisis concerning the fact]].
** According to the ''Final Hours of Portal 2'', Peer Review was going to contain a single player campaign as well. The original plan was for players to be either ATLAS or P-body, who would be recruited by Rick to stop a brain-uploaded Cave Johnson from taking over the Enrichment Center. [[SequelHook Valve decided it would require too many resources for ''just'' a DLC...]]
** According to the developer commentary, the reveal that the series takes place in the ''VideoGame/HalfLife'' universe wasn't the original plan, but was ultimately incorporated into the story to make [[PropRecycling the reuse of assets]] from ''VideoGame/HalfLife2'' less jarring.