"Gotta catch 'em all!" was a Pokémon slogan, and that's precisely what a lot of players tried to do. But unlike the mythical Yoshi, there was one real Pokémon who was very hard to attain: Mew, Pokémon number 151. In "Pokémon Red" and "Pokémon Blue," rumor had it that you could unlock Mew by surfing to a particular shore that had a hidden pickup truck, doing something to the truck to reveal Mew (possibly including slashing the truck's tires with the cut ability and pushing it with the strength ability, or unlocking the truck with a card key) and fighting Mew when it emerged.
The lone pickup truck did exist in a hard-to-access location, likely fueling speculation that it had to be there for a reason. It was near the ship S.S. Anne in Vermillion City, but it wouldn't budge. The truck was actually in an area that players were never likely to see; you could only get to it by traversing water, and the surf skill was available in the game only after boarding the S.S. Anne. But Game Boy allowed you to hook up your device with another Game Boy and swap characters with friends, so you could receive a character with the ability before reaching that point in the game and surf on over to find the apparently secret truck.
As was mentioned above, some gamers did legitimately have the character Mew, but they got it by attending a Nintendo event and linking up their Game Boy with that of a Nintendo representative (the only official way to get Mew). Mew could also be obtained by hacking your Game Boy with a Gameshark, Game Genie or Pro Action Replay device and using a cheat code. But alas, Mew was nowhere to be had in the actual unadulterated game.