YMMV / Portal

• Common Knowledge: Due to "The Cake Is a Lie" becoming an abundant meme to the point of being long discredited, a lot of people who haven't played Portal take the phrase at face value and believe there is no cake in the game. As a result, first-time players might be surprised when The Stinger shows that there really is a cake somewhere in Aperture, supposedly baked by order of GLaDOS; the lie is that she had no intention of giving it to the Player Character.
• Demonic Spiders: The turrets in Advanced Chamber 16 are stuck in cages, rendering them impossible to move or deactivate.
• It Was His Sled: The biggest plot-related spoiler is really that there's a plot at all, since at first glance the game appears to be just a fairly straightforward puzzle game. But with a sequel game (that has an advertised plot) and GLaDOS becoming a well-known character outside of the game, it's more likely that you'll be going into it knowing the main plot twist.
• Magnificent Bastard: GLaDOS is computer created by Cave Johnson to run Aperture Science after his death. Once activated, GLaDOS proceeded to kill most of the scientists with neurotoxin and took over the Enrichment Center to conduct tests at her own leisure. In the present, GLaDOS put Chell through a series of potential lethal tests before trying to murder her. Defeated and shut down by Chell in retaliation, GLaDOS is accidentally awoken years later by Wheatley. Throwing Chell back into more tests, she is eventually overthrown by Wheatley thanks to Chell's actions. With Wheatley threatening to destroy the facility with his own stupidity, GLaDOS convinces Chell to help her take Aperture back from Wheatley, and in turn lets Chell go when that is done. Switching focus toward Cooperative Testing, GLaDOS uses the robots Atlas and P-body to secure control over the lower parts of the facility and eventually give her access to more human test subjects. When a mysterious hacker (actually a mother bird on a keyboard) forces her hand, resulting in the deaths of her human test subjects, GLaDOS sends the robots after it before adopting the baby birds to raise as killers.
• That One Achievement: The gold medal challenges are very unforgiving. Just the "least portals" requires exploiting game's physics to skip some of the puzzles, and it only gets worse from there.

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