Dragon Ball is particularly infamous for its use of filler, where fights are artificially lengthened and extra scenes of little consequence are added so that the anime does not "overtake" the manga. However, not all filler has to be this way; sometimes it can fill in gaps left behind by Akira Toriyama, or it can provide some extra depth to something only briefly touched upon in the manga.
But what happens when Toriyama goes back and fills in those gaps himself, or if the anime writers missed a detail that contradicts something they just introduced? Accidents will happen, especially with hundreds of manga chapters and episodes to work with, and there are several examples of this happening throughout the Dragon Ball series.
8 Dr. Flappe
Android 8 was the first of the Red Ribbon Army's numbered androids to appear, being introduced in episode 39 of the original Dragon Ball. Much like Android 16, he is a gentle giant who only uses violence as a last resort. In episode 42 he and Goku journey to get his self-destruct device removed so that he can live in peace, eventually consulting with his original creator Dr. Flappe to do so.
It's understandable that the anime writers wouldn't expect Akira Toriyama to return to the Red Ribbon's androids hundreds of chapters later, revealing they were created by a character readers had never seen before called Dr. Gero. Some supplementary material tries to reconcile this by claiming Flappe co-created Android 8 with Gero, but his absence from all later Dragon Ball media suggests they'd rather he'd be forgotten.
7 Piccolo Destroys Goku's Space Pod
After recovering with a freshly-grown Senzu Bean in episode 46, Goku makes plans to head to Namek himself in a spaceship built by Dr. Brief. He explains that, since Nappa and Raditz's space pods were destroyed, they instead salvaged the space pod that he himself was sent to Earth in as a baby. Too bad Piccolo completely obliterated it back in episode 18.
During Piccolo and Gohan's training before the arrival of the Saiyans, Gohan transformed into a Great Ape again after his tail grew back and a full moon appeared in the sky. Piccolo had already destroyed the moon to stop Gohan's first transformation, so it turns out that this new moon is just a projection created by Goku's long-dormant space pod. Piccolo blows it up and Gohan returns to normal. To be fair, the anime inserts some dialogue from Dr. Brief saying that the pod was in a sorry state when they found it, but badly damaged is a far cry from totally destroyed!
6 Gohan's Cell Nightmare
A lot of new material was added to the anime detailing the Saiyans' time inside the Hyperbolic Time Chamber. Episode 167 features a scene where Gohan's training is interrupted by his mother Chi-Chi and Piccolo. Suddenly, Cell appears in his newly perfected form and murders them right in front of him. Gohan wakes up screaming, the entire event being just a nightmare.
The problem here is that Gohan has no idea what Cell looks like. Several episodes ago in episode 156, this fact was made very explicit: Goku suggests that Gohan imagine his friends and family being hurt by Cell to enrage himself enough to become a Super Saiyan, but Gohan reminds him that he hasn't even seen Cell before. In response, Goku tells him to substitute Cell for Frieza instead.
5 Vegeta Knew Frieza Destroyed The Saiyan's Planet
In episode 104, while Super Saiyan Goku and Frieza enter the final stage of their battle on Namek, Vegeta is on Earth with the rest of the cast. He flashes back to a time when he was still working with Nappa and Raditz. A fuming Nappa tells Vegeta that it was Frieza who destroyed their home planet, but Vegeta reveals that he knew this secret all along.
This flies in the face of Vegeta's behavior previously during the Namek arc, where both he and the audience are first told about Frieza's actions by Dodoria. Vegeta is initially surprised, and it is clear that this is completely new information to him. Likewise, Raditz was present when Nappa revealed the truth, yet still recites the old cover story about an asteroid to Goku when the two meet.
4 Chi-Chi Already Knows How To Drive
This is relatively minor compared to other inconsistencies, but it involves one of the most well-known filler episodes in Dragon Ball Z: episode 125, where Goku and Piccolo learn how to drive. It's a favorite among the fandom, providing some levity between Trunks's foreboding message from the future and the arrival of the androids.
What's strange about this episode is that, despite Chi-Chi being the one who demands the two learn to drive to help with errands, she already appears to know how to drive. Occasionally she is seen in the driver seat of a hovercar, such as when she arrives as Kame House to find out what happened to Goku and Gohan in episode 8. Then again, if they had remembered that tiny detail, then fans never would've got to see Piccolo's "POSTBOY" shirt.
3 King Kai's Saiyan History Lesson
In episode 20, after arriving at King Kai's planet and beginning his training, Goku gets a history lesson about his Saiyan ancestors. Many details from the story would end up being disproven: King Kai claims that Planet Vegeta's own guardian destroyed the planet, when it is eventually revealed that it was Frieza's doing, and the Saiyans stealing the Scouter technology from the Tuffles is contradicted much later in Dragon Ball Super: Broly, where it is explicitly a Frieza Force invention.
One aspect that is often overlooked is when King Kai mentions the Saiyan's ability to turn into Great Apes under a full moon. This ought to be news to Goku who, outside a vague allusion from Raditz, would not learn about this until Vegeta becomes one in episode 32. On seeing it, Goku realizes that he himself was the "monster" that killed Grandpa Gohan, but when King Kai mentions this he doesn't put two and two together.
2 The Nature Of Hell
Dragon Ball has always played pretty fast and loose with its mythology, adding and removing realms and deities whenever the story demands it. While alluded to in the manga, Hell is never physically depicted, so the anime decided to fill this gap by having Goku end up there by mistake in episode 13. It doesn't seem like such a bad place, with parks and carnivals that make it seem more like a vacation spot than a place of punishment. Villains like Frieza and Cell are seen there in other episodes, still in possession of their physical bodies.
It isn't until much later in episode 237 that Piccolo explains to Majin Vegeta what happens to evil souls: they are separated from their bodies, purified and reincarnated with no memory of their past life. This is ultimately the fate of Kid Buu at the end of the arc. That's not to say later stories have been consistent with this; in Resurrection ‘F’, Frieza is shown in a version of Hell with his body still intact, so the exact nature of Hell in the Dragon Ball universe is not set in stone.
1 The Entire Garlic Jr. Arc
One of the more infamous stretches of filler in Dragon Ball Z, the Garlic Jr. arc takes places between episodes 108-117, after Frieza's defeat on Namek and before Goku returns to Earth. Gohan, Piccolo and Krillin have to contend with their old foe Garlic Jr., who has attacked Kami's Lookout, imprisoned him and used the Black Water Mist to turn almost everyone into savage vampires.
Garlic Jr. originated from the first Dragon Ball Z film Dead Zone, and anyone who knows about the canonicity of the theatrical movies will understand why this arc doesn't work. It appears to be set before the arrival of Raditz, but by then Krillin had not met Gohan and Goku hadn't seen Piccolo since the 23rd World Tournament. There isn't anywhere in the timeline that the events of Dead Zone could comfortably slot in, making it impossible for the Garlic Jr. arc to occur.