Urban Legend Of Zelda / Undertale

Undertale attracts a lot of this, due to its reputation for large amounts of Developer's Foresight.

• W.D. Gaster is an intentional nod to this trope, Glitch Entities and Dummied Out characters. Every reference to him in the game is only accessible by tampering with the game settings or through incredible luck, and together they paint a disturbing picture of a brilliant scientist who was shattered through time and space by falling into one of his machines, erasing him so utterly that it's like he never existed at all. Gaster seems intricately connected to the plot of the game — especially to Sans, who just might be the one person who still remembers him, but you would never know that if you didn't dig deep below the surface of the game. In short, his very existence seems to be tailor-made to incite fan speculation about his nature, his death, and his role in the events of the game.
• A rumour so prevalent that it's posted on many fan sites as fact: The strange-looking, skeleton-like NPC in a room that has a 1/1000 chance of spawning per run who disappears when you examine it is almost universally said to be W.D. Gaster, even though this has not been confirmed by Word of God and the game files identify the character as "spr_mysteryman_0".
• An example of what could be called an Ascended Urban Legend of Zelda: the demo of the game features a Toy Knife as an equippable weapon, so a rumour started that by playing violently, you can find a Real Knife instead. Toby himself disproved it at the time so people wouldn't waste time looking for something that wasn't there...and then turned around and added said item to the actual game. The Real Knife replaces the Worn Dagger, the strongest weapon in the game, on a Genocide run, which indeed is playing violently. The description for the Real Knife says, "About time". Problem is, by then you have no need for it, since the only battle left after you acquire the knife is an endurance match where standard attacks are ineffective and the final blow is scripted.
• The game has several unused music tracks. Some of these music tracks have rumors surrounding them — that "kingdescription" was used for an early version of the scene where you meet Asgore◊, and that "ruinspiano" was meant for a longer version of the ending showing the monsters leaving the Underground. In reality, all of these were invented by a fan to promote their animation project; when it fell through, they confessed they had made them all up. This hasn't stopped some people from swearing up and down that this is absolutely what they were intended for.
• A strange case of something true that's sometimes mistaken as false: The unused song "Star" was meant to be used when Madjick's attack orbs turn into hearts, as stated by Toby. However, because of the above email hoax, this message is often said to be fake too, despite being an unrelated email posted earlier by a different user. (In fact, the fake email was likely an attempt at copying the real one.)
• A somewhat malicious example: There's a rumor going around saying that the semi-hidden encounter So Sorry can be killed without hampering a Pacifist run. It's also rumored that the same goes for Jerry. This isn't true, and might have arisen from the fact that neither of them must be killed for a Genocide run; in fact, So Sorry actually cannot be encountered in a Genocide run.
• Another disproven rumor involving the Cloud Cuckoo Lander Temmie Shopkeeper stated that if you refused to sell her an item when she offered an increased amount of money for it twice in a row, she'll drop her facade and tell you that you will regret it in proper English; as a result, all Temmie encounters from that point onwards would cause a One-Hit Kill if you got hit by any of her attacks. While the first part is true, the One-Hit Kill effect isn't.
• Soon after the game's release, a few spooky hoaxes regarding the Fallen Child and Dr. Gaster had been made by fans (such as the Fallen stalking the protagonist or Gaster corrupting everyone as is typical of creepypastas), but given how quickly the game was datamined for research on its secrets, it didn't take long for those pranks to be disproven. Oddly enough, while you can't get the Fallen to appear in reflections, there is an unused effect in Waterfall that changes the protagonist's reflection to one who's cast in shadow and wearing the Fallen's colors.
• There's a rumour of a hidden Temmie ending. It doesn't exist and the idea has either been spread by somebody trying to have a chuckle or people who misunderstood the dog ending.^note The "small white dog" is not Temmie, but Toby/the annoying dog.
• The most depressing Neutral ending, the one you'll probably get if you play through the game like a regular RPG and never touch the Mercy button, only actually requires you to kill every boss and a large number of normal enemies (but not all of them). The game doesn't acknowledge "never touching the Mercy button" in any way, and you can get this ending even if you Spare some enemies, as long as you also kill enough of them. The confusion has caused some players to resist referring to the worst ending as "No Mercy" (hence the alternative term "Genocide Route"), on the grounds that that term needs to be reserved for this ending.
• Tumblr has a really bad habit of creating what is, essentially, fanart in the form of mock screenshots, and then presenting it as a "newly-discovered secret", which is then believed wholeheartedly for various reasons until the dataminers finally debunk it. Examples include Grillby and Sans having hidden shop interfaces, fighting Grillby as a Route Boss in a No Mercy run, the game turning into an abridged shout-out to Cave Story if you name the fallen child "Quote", Muffet webbing up Sans and you having to cut him free, and many, many more.
• The last and most bittersweet moment of the Pacifist ending, Asriel's ultimate fate, has had multiple rumors and methods spin around social media about how you can save him. These range from easy and sensible (saving the butterscotch pie and eating it near him), to the absurdly difficult, like completing a No-Damage Run. None of these have been found to be true; there's nothing that can be done for him. ^note Saving the butterscotch pie for the end does trigger a unique response for Asgore, who debuffs temporarily due to recognizing the pie, but does nothing special for Asriel (unless you eat it during the Asgore & Toriel Lost SOUL sub-fight).
• For a while, fans were stuck on what triggered the "Queen Alphys" ending of the game; specifically, just what makes Mettaton NEO give his "you're not completely evil" speech instead of commenting on how you don't want to be in his fan club. While the clear cause is dropping the No Mercy path at the last chance possible, what triggers it had been attributed to trying to Spare Mettaton NEO before attacking, missing attacks or fooling around for a few turns, or by attacking him and not getting a near-perfect hit. However, it was eventually defined that the only way to trigger the sequence was to simply not kill everyone in Hotland/Core before challenging Mettaton.
• After completing the No Mercy path, the world is destroyed and you have to sell your SOUL to get it back. This caused many players to, somewhat logically, assume that completing it again would break the game irrevocably, since you would no longer have a SOUL to sell. It doesn't, though; you just skip the required action entirely on subsequent runs.
• There was a rumor based on bad information that the Genocide/No Mercy final boss pulling an "I Know You're in There Somewhere" Fight on you before offering to spare you is specifically after you reset a pacifist run to do a No Mercy run, and that it's because he remembers the Pacifist ending despite a True Reset wiping everything. In actuality, the "I Know You're in There Somewhere" Fight speech happens regardless of how many other endings you've seen (including no endings).
• When Sans gets serious and changes his font for one line, many fans believe the font in question is named "Sans Serif". "Sans Serif" is not a name of a font, but a type of font, and all of Sans's speech is in a sans serif font. Hell, it's what the Sans in Comic Sans stands for.
  • To be more specific, a serif is an extra line or stroke added to the main stroke that composes a letter, and "sans" means "without". Times New Roman is a serif font, Helvetica is a sans serif font.
• It was initially believed by many people that the phone call in the Ruins where Toriel asks you about allergies was a Fun event because they noticed that it didn't happen on every playthrough. However, it was soon found out that this was not affected by Fun value, but instead the result of a bug; checking the sign in the room where you're supposed to receive the call will prevent it from happening, and people playing the game for the first time were more likely to fall into this trap than if they had already played before and not felt the need to read the sign.
• It didn't even take an hour after Toby put out a 116 megabyte update to fix bugs before fans tore into the game's code looking for more stuff related to Dr. Gaster. While nothing's been found as of yet (and Toby made a post apologizing for no new content, though that doesn't necessarily rule out Gaster stuff), this one's fairly understandable because putting in content and hiding it as a bugfix is definitely something Toby would do. As it turns out, there is new dialogue regarding Sans' room and abandoning a No Mercy run on Papyrus, which isn't new content, but a change to preexisting content.
• After surviving the Genocide final boss's opening attack once, if you have to refight it, it'll interrupt the opening dialogue to catch the player off guard. This will happen every time you start the battle having survived the first attack exactly once (surviving it a second time changes his dialogue again), contrary to a persistent belief that it does this to punish you for trying to skip dialogue with X.
• Naming your file "Chara" does not disable your ability to rename after a regular reset — you don't have that ability in the first place, unless you named your file "Frisk", and even that's only an Obvious Rule Patch to prevent you from being stuck in Hard Mode.
• For whatever reason, Toby used real beta assets (old versions of Frisk's sprite and the Enemy Approaching track, as well as a probably-real battle screenshot of several Froggits, then called TestMonster) to make a joke post on starmen.net, claiming they were for the EarthBound (1994) ROM Hack sequel to the subject of the thread... which was itself a nonexistent joke sequel to another hack. Those not familiar with the EarthBound hacking community failed to recognize the thread as a joke, and took the post as serious insight into the development process. Toby eventually went back and blanked said posts because so many people kept insisting they were legitimate.
• After the update patch was released, a very persistent rumor circulated the web for a while, claiming that due to a bug, once the patch was installed, it would be impossible to go back and talk to Asriel after the final boss. This is false: once you've reached the surface and watched the credits, it's always impossible to talk to Asriel by reloading your file, even if you don't have the patch installed. This misunderstanding spread because some people tried to reload their finished save and visit Asriel after installing the patch, not knowing that it had always been impossible by game design.