So pervasive was Pokemon in the late '90s and early 2000s that it's easy to assume the franchise was an immediate hit right from the outset--but that wasn't exactly the case. As it happens, the original Pokemon Red and Green versions actually got off to a rather inauspicious start when they first launched in Japan in 1996, two years before the property made its international debut. Initial sales were so sluggish that developer Game Freak worried that the games might have missed the proverbial "last train"--and that could very well have been their fate were it not for a secret Pokemon hidden within their code.

　　Development on Pokemon Red and Green began in 1990, only one year after the Game Boy itself had launched, but their gestation was infamously protracted. The games would take nearly six years to complete, and by the time they were finished, the Game Boy was verging on obsolescence. Making matters worse, Red and Green missed the all-important holiday release window and slipped into the early part of the next year--a traditionally slow period for game sales.

　　"We originally completed the titles in October [1995] and wanted to release them quickly. But we missed the end-of-year sales season and finally released the games at the end of February of the following year--the very worst time of year to release games!" Pokemon Company CEO Tsunekazu Ishihara recalled in an interview with late Nintendo president Satoru Iwata.

　　Thanks to this unfortunate timing, Red and Green's early sales were slow. "It really was a quiet start," Ishihara said of the launch. Gradually, however, the games' fortunes improved. Sales numbers steadily picked up as positive word of mouth spread and the games received more media coverage, but they wouldn't truly take off until players discovered a secret hidden in the titles: a 151st Pokemon named Mew.

　　Unlike other "legendary" Pokemon such as Mewtwo, Mew does not appear in Pokemon Red and Green during the normal course of the adventure; it could only be acquired by outside means, which in Pokemon Company parlance makes it the series' first "mythical" Pokemon. The reason for this is, in part, because Mew was actually a last-minute addition to the games, programmed in at the eleventh hour--without Nintendo's knowledge--as a "prank."

　　"We put Mew in right at the very end," Shigeki Morimoto, who worked as a programmer on Pokemon Red and Green, explained to Iwata. "The cartridge was really full, and there wasn't room for much more on there. Then the debug features which weren't going to be included in the final version of the game were removed, creating a minuscule 300 bytes of free space. So we thought we could slot Mew in there. What we did would be unthinkable nowadays!"

　　Although Mew's existence was hinted at in the games, particularly in the journal logs around Cinnabar Island recounting Mewtwo's birth, Morimoto explained that the Pokemon was never intended to be discovered--and it could very well have remained hidden from the public eye had some players never chanced upon it.

　　"Unless we could think about any good opportunity to do so, the existence of Mew wouldn't have been revealed to the public. It was left in there so it would be ready in case it suited to some post-launch activity to make use of it," Morimoto said. "But if there wasn't anyone among ourselves who wanted to use it, I thought it would be fine to just leave it as it was."

　　Despite being secreted away, it wouldn't take long for players to inadvertently stumble upon Mew. Through an exploit, some users managed to encounter the hidden Pokemon in the wild. "[D]ue to an unforeseen bug, Mew ended up appearing in some players' games. It looked like we had planned all of this, but that wasn't the case," Morimoto explained.

　　This discovery spawned numerous rumors about how to obtain Mew, many of them revolving around the mysterious truck in Vermillion City's harbor. Parked on an inaccessible patch of land in the corner of the area, the truck was the subject of much intrigue due to its conspicuousness; it was the only object of its kind in the games, and it could only be reached using Surf, a move that wasn't obtained until much later in the adventure, long after players could no longer (legitimately) return to the harbor.

　　Unless we could think about any good opportunity to do so, the existence of Mew wouldn't have been revealed to the public."

　　Shigeki MorimotoRumors that Mew was hidden under this truck soon circulated across playgrounds and message boards. Of course, as is so often the case with playground lore, these rumors turned out to be unfounded. The truck was ultimately nothing more than a static piece of scenery; even if players could reach it, they could not interact with it in any way. Still, the intrigue surrounding Mew and how to obtain it led some players to contact Nintendo--which was not even aware of the Pokemon's existence since it was inserted secretly after the debugging process was complete.

　　To stymie any possible repercussions from this wayward prank, Pokemon creator Satoshi Tajiri proposed formally unveiling Mew. This led to an official promotional campaign. In April 1996, CoroCoro Comics announced the "Legendary Pokemon Offer." Twenty winners would get the opportunity to send their Pokemon cartridges in and have Mew officially unlocked in their games. The promotion was just the sort of jolt that Pokemon Red and Green needed. More than 78,000 people entered the contest in the hopes of receiving a Mew, and game sales exploded.

　　Both Iwata and Ishihara attributed Pokemon's remarkable rebound to the success of the Legendary Pokemon campaign. "There was a really incredible response to CoroCoro Comic's announcement of the Mew offer. I feel that's really when things turned around for Pokemon," Iwata said.

　　Ishihara agreed: "I believe so too. The monthly sales we'd had up to then began to be equaled by weekly sales, before increasing to become three then four times larger."

　　By the time Pokemon reached other shores in 1998, it was already a multimedia empire spanning an animated series, a trading card game (which would become a phenomenon in its own right), a toy line, and other licensed merchandise, and it wouldn't take long for other parts of the world to likewise be swept up in Pokemania. After Red and Blue's release, similar Legendary Pokemon promotions were held in other regions, giving players outside Japan the chance to send in their Pokemon cartridges to receive their own Mew.

　　After 25 years, the Pokemon franchise remains a veritable juggernaut. Its most recent installments, Pokemon Sword and Shield, have collectively sold more than 20 million copies since their 2019 launch, making them the first pair of Pokemon games to break that milestone since 2000's Gold and Silver. Even more than two decades on from its debut, the Pokemon series is still going strong--and it owes much of its success to the little prank that Game Freak snuck into the original games under Nintendo's nose.