Ancient Mew is such a cool Pokémon card, but did you know that it was based on a normally-unobtainable version of Mew programmed into Pokémon Red, Blue, and the Japanese Green (its coding doesn't exist in Pokémon Yellow because the extra data created by Pikachu's "real voice" corrupted the data)? Someone unknown programmed it into the game without permission during the last phase of development. Other developers didn't notice it until they started reworking the data to make Gold and Silver, when they finally removed it. They later thought to bring it back as a Legendary Pokémon obtainable from one of the statues in the Ruins of Alph (which went by a different name at the time, as the Unown were going to be non-Pokémon runes, since those already existed in the coding), but instead decided to make it into a TCG-exclusive card.
Some of you who like to play with glitches may have noticed that the characters in the player's name affects the Pokémon that appear on Cinnabar's east coast. MissingNo. appears when you talk to the old man because the game temporarily treats OLD MAN as your name. In order for Ancient Mew to appear, your name must be YAILPHX.
Be sure to catch it before it gets a chance to attack, because all of its moves are glitchy and will make your game freeze, potentially harming your save data. This is likely because the person who programmed it into the game didn't have time to give it a proper move pool (it probably took too long to create the extra text table that contains the runes). Due to its ridiculously-high catch rate, only a Master Ball will work. After catching it, the Pokédex attempts to register it, but due to improper coding, it blanks out for about 30 seconds, then loads a page full of question marks and this glitch/trick is fake (sorry, guys). Like Mew, it can learn all TMs and HMs, so be sure to delete those harmful glitch attacks right away!