The MissingNo in Pokémon X & Y has finally been explained.
Pokémon X & Y were released in 2013 and still have a few unanswered mysteries, such as the ghost girl who tells the player that they are "not the one." It only took six years, but one of the mysteries of Pokémon X & Y has finally been answered - does MissingNo appear in Lumiose City?
MissingNo is an infamous glitch from Pokémon Red & Blue that involved triggering a random encounter in an area with no wild Pokémon data. The game would take data from nearby sources, such as the player's name, and create a Pokémon from scratch, which would often result in a bizarre mixture of graphics, sound, and stats. The most famous form of MissingNo resembles an L-block from Tetris, even though there are many different kinds of MissingNo, some of which can use graphics from creatures that aren't Pokémon, like the fossils or the unidentified ghost.
In 2013, websites like Kotaku posted articles about a strange occurrence that players had noticed in Lumiose City. If the player stands near the front door of Boutique Couture and tries to manipulate the camera, then it's possible to see a shape that resembled the L-block appearance of the most famous form of MissingNo. It turns out that the appearance of MissingNo is, fittingly enough, the result of a graphics glitch, which has been discovered by YouTuber Shesez (of Boundary Break fame) who detailed the MissingNo glitch in his latest video.
According to Shesez, the shape in the doorway that resembles MissingNo is actually the doorway for a coffee shop on the other side of the building. For whatever reason, the outline of the coffee shop's doorway bleeds through the graphic of the Boutique Couture doorway. The reason the coffee shop doorway resembles MissingNo is that there is a small sign with a cup of coffee near the top left of the doorway, which cuts off part of the image.
It seems that Game Freak had not included a secret reference to the most famous glitch Pokémon in Pokémon X & Y. It turns out that the Pokémon series just has a weird predilection towards glitches that resemble Tetris blocks.