Even many non-fans of Dragon Ball Z have heard the famous "it's over 9000!" line, however not everyone realizes that this phrase is actually a mistranslation from the original Japanese anime. Taking place during Dragon Ball Z's Saiyan saga, the scene in question comes when Goku returns from his training with King Kai to confront Vegeta and Nappa, as they strive to conquer Earth and steal the wish-granting titular balls. Goku begins powering up, showing off the fruits of his labor, and Nappa asks what Vegeta's scouter measures Goku's power level to be. In a moment that would eventually take on a life of its own, the Saiyan prince exclaims with unbridled rage, "it's over 9000!"
When this scene first aired in 1997, fans were focused squarely on Goku's awesome strength, but in more recent years, this moment has become a meme used across the internet on a daily basis to describe when something reaches an absurdly high level. The meme began to spread in late 2006 on a variety of message boards and has endured in popularity ever since, attaining a mainstream presence even among those who are unfamiliar with Dragon Ball Z. The franchise spawned a second viral meme with Frieza's "this isn't even my final form" line.
However, Vegeta's line was never supposed to be "it's over 9000!" and in the original Japanese anime, the Saiyan actually cries, "it's over 8000!" during this scene. The 8000 version is also used in Akira Toriyama's original Dragon Ball manga series and is translated correctly in the English edition of the volume. The line's incorrect translation came about when the Ocean Group were tasked with dubbing the original Japanese Dragon Ball anime into English in the 1990s.
Opinion differs as to whether the switch from 8000 to 9000 came about purely by error, or whether the change was made intentionally. On one hand, the Daizenshuu 7 book claimed that saying "9000" in English was a better fit for Vegeta's animated mouth movements, however it's also important to note that Dragon Ball Z's Ocean dub was notorious for making translation errors (such as Goku believing Vegeta killed Grandpa Gohan) which can't be explained away so easily. Interestingly, however, when Dragon Ball Z was re-dubbed by Funimation with the now familiar voices of Sean Schemmel and Christopher Sabat, the mistranslation was retained, and has featured in most English re-dubs and video games ever since, while in non-English speaking territories, the 8000 version is predominantly used.
In terms of the effect this mistranslation has on the Dragon Ball Z story, there are certainly other errors that have a more detrimental impact. The switch from 8000 to 9000 does imply that Goku is a little stronger than he should be, but with Dragon Ball power levels soon to rocket into the millions, the difference ends up being inconsequential. It could even be argued that the translation issue drew attention to Dragon Ball Z's "it's over 9000!" scene and contributed to the line becoming one of the internet's favorite phrases.