Why doesn't the One Piece anime reveal the cause of Monkey D. Luffy's famous facial scar at the same point as the manga? Created by legendary mangaka Eiichiro Oda, One Piece set sail in 1997. Over one thousand chapters and 980-or-so episodes later, the franchise is still going strong, as Monkey D. Luffy's Straw Hat crew continue their search for Gol D. Roger's fabled treasure and the mythical island Raftel (or, indeed, Laugh Tale). As One Piece viewers of any experience level will know, Luffy is an impulsive sort, even for a Shonen Jump protagonist. He's the type to jump into battles headfirst, throw his friends off cliffs, and poison himself just to prove a point.
It's hardly surprising that Luffy has a few battle wounds, then. The large X-shaped mark on his chest was added following One Piece's two-year time skip - a lingering gift from Akainu at the Battle of Marineford (though some maintain the wound was inflicted during a filler episode) - and Luffy has a curious resistance to poison after his noxious battle with Magellan at Impel Down. But Luffy's most prominent scar is undoubtedly the small curved cut beneath his eye, present from the very beginning of One Piece.
For many anime-only viewers, the cause of this scar may be a complete mystery. One Piece initially neglected to show how Luffy came to have the mark upon his face, leaving viewers to assume it was simply the result of a childhood misadventure. Manga readers, however, will know exactly how Luffy's cheek got blemished. In One Piece's very first chapter, a young Monkey D. Luffy is trying to convince Red-Haired Shanks to accept him as a crew mate, with the then two-armed captain merely laughing at this child's dreams of piracy. To prove his commitment and toughness, Luffy takes a knife and stabs himself right under the eye, leaving Shanks' crew horrified. Luffy is patched up, everyone drinks to his cavalier attitude and, not long after, the future "fifth Yonko" eats the Gomu-Gomu Devil Fruit. For the anime, however, this scene came much later - potentially partially to keep the mystery, but likely also because the anime didn't want to risk using the scene so early in its run.
In the anime, One Piece episode 1 begins in the present day, with Luffy rescuing Koby from evil pirates and encountering his first mate, Roronoa Zoro. The Shanks flashbacks don't begin until episode 4, and they skip over the whole face-stabbing incident entirely. Why? Probably because a child shoving a knife into their eye isn't the best image to be putting out in an anime. Japanese animation is well known for being more relaxed in its content compared to cartoons in the west, with Sanji puffing away on cigarettes and plenty of blood spilled during fight scenes. Even so, a child knifing themselves in the face during episode 1 was potentially too risky a move for the show to reveal this particular detail until Luffy's risk-taking behavior clearly demonstrated his act wasn't to be repeated.
If this is the case, why was the scene included in the One Piece manga? Traditionally, comics in Japan are even more lenient than anime when it comes to violent or mature content. Fortunately, the One Piece anime made up for the dropped scene - albeit only after Eiichiro Oda's franchise had already established itself as an industry staple. In 2012, the Episode of Luffy special was released to coincide with One Piece Film: Z, separate from the main anime. This feature-length animation retold Luffy's origin story, this time including the eye-stab, which had gone untold on-screen for so long. The scene would be included in later flashback sequences, but only much more recent offerings, such as episode 878 in 2019.