After Wheatley takes over GLaDOS's body, he blurts out a sentence in Spanish and says that even he doesn't understand it. The English translation of the sentence is, 'You are using this translation software incorrectly. Please consult the manual.'
It is not known exactly how long Chell remained in stasis between Portal and Portal 2. At the beginning of Portal 2, she is awakened for the first time after 50 days of sleep for mandatory physical and mental exercises. On her next awakening, the announcement system gets stuck repeating the number 'nine,' which could mean she slept for at least 273 years or that the system reached its counting limit. The facility falling into ruin after GLaDOS's defeat at the end of Portal could also point to the announcement breaking down and the period being shorter. However, the book 'The Final Hours of Portal 2' states that this period was 50,000 years.
In a newspaper clipping, the lines 'Local entrepreneur buys salt mine' and 'Cave Johnson brings science and industry to Upper Michigan' reveal the location of the Aperture Science test laboratories.
The game was initially planned as a prequel to Portal rather than a sequel. A character named Mell would take Chell's place, Cave Johnson would appear in person rather than only in voice recordings, and a new style of gameplay would be invented in which portals were never mentioned. Negative feedback from playtesters pushed the development team to make a direct sequel.
GLaDOS's name may have been inspired by Dr. Gladys J. Loring, one of the psychiatrists who evaluated Mercury astronaut candidates at Wright-Patterson. These candidates were subjected to extreme experiments that pushed the limits of ethical testing.
Some test chambers contain hidden rooms known as Rat Man's dens. Each den contains one of the wall paintings he made.
The saying 'it ain't over till the fat lady sings' is based on the cliché of large-bodied female singers taking leading roles in opera performances. Staying true to this saying, Portal 2 ends with an opera song sung by a turret that has a uniquely fat and feminine voice.
Before the game's release, numerous edits were made to the original Portal both to tie in with Portal 2 and to plant clues. The most notable change was the ending being rewritten so that Chell cannot escape the facility and is dragged back inside by a robot after defeating GLaDOS. Radios were also added to all test chambers; when placed in specific spots, these radios emitted SSTV sounds, a primitive form of television broadcast, presenting various promotional images related to Portal 2. A similar SSTV surprise is hidden inside Portal 2 as well.
In one of the game's commercials, in which J.K. Simmons voices Cave Johnson, an exploded diagram of the Portal Gun briefly appears. One of its central components is a miniaturized Black Hole.
Cave Johnson's face is based on the appearance of Bill Fletcher, Portal 2's Lead Animator, while his assistant Caroline was inspired by Valve Visual Designer Laura Dubuk. In a hidden room, a portrait of the two can be seen, made by another Valve artist, Jeremy Bennett (Caroline's appearance was artistically aged, since Dubuk is much younger than her in-game counterpart).
The PC version of Portal 2 contains a classic secret. While playing fan-made levels published through the Steam Workshop, the background music is often an electric remix of Johann Sebastian Bach's Prelude No. 1 in C Major.
Throughout the game, Wheatley and GLaDOS never address or call each other by name.
Wheatley was planned to die when GLaDOS wakes up, to show how evil and depraved she is.
Rat Man (or someone else killed by GLaDOS) appears for only ONE frame; if you pause at just the right moment while in the elevator in the final scene, you can see a silhouette.
After Chell has been in suspension for 50 days, when the audible number 9,999,999 is added (assuming this is the upper limit the computer can count to), Chell has slept for AT LEAST 27,397 years, 4 months and 24 days. The real number could be much higher.
While the developers were trying out different ideas for how the game could end, they tried having the player press the middle mouse button so that Chell says 'yes' for the first time and solves the core transfer lock. However, this idea was shelved because the developers found it frustrating.
There is an unused monologue belonging to GLaDOS that was planned to be in the game but was cut because playtesters found it too disturbing.
Initially, Wheatley was going to be killed by GLaDOS when she is accidentally awakened again. As Chell then made her way through the facility, she was going to encounter multiple Personality Cores that would help her escape Aperture and defeat GLaDOS, who would become the main antagonist. However, playtesters loved Wheatley and demanded he be brought back; as a result, the second half of the game was rewritten to include Wheatley and turn him into the secondary antagonist.
There is a persistent theory among the fanbase about Chell's past. Many claim that Chell is actually the daughter of Cave Johnson and his secretary Caroline, and that she was kept in stasis continuously for years as the last test subject. The English translation of the closing opera 'Cara Mia Addio' reveals that GLaDOS addresses Chell as her daughter, supporting this theory. Others suggest that the Chell in the game is a clone of the real person who died during GLaDOS's neurotoxin attack on Bring Your Daughter to Work Day. Valve has made no official statement about Chell's past.
In the 'Escape' chapter, Chell can find a defective turret being taken away for disposal. If she manages to take it, the turret talks about the myth of Prometheus, who was buried beneath Olympus to be pecked by birds as eternal punishment. It also says disjointed lines such as 'Make lemonade!', 'Go crazy!' and, most importantly, 'Her name is Caroline' and 'The answer is beneath us'. All of this connects to the next chapter, 'The Fall', in which Chell and GLaDOS (in a potato battery) are sent by Wheatley to the abandoned lower depths of the old Aperture facility, and GLaDOS is grabbed and pecked by a bird. Chell also discovers Caroline, Cave Johnson's secretary and GLaDOS's human trace.
In the 'The Fall' chapter, if Chell approaches the abandoned underground Aperture Science test facilities, the background music turns into a melody similar to the main theme of the Spider-Man trilogy. Immediately afterwards, Cave Johnson's voice is heard from the speakers. Cave Johnson is voiced by J.K. Simmons, the actor who played J. Jonah Jameson in the Spider-Man films.