Animation is expensive. Really expensive. An average 22-minute episode of an anime costs around $123,000, and American shows tend to be double that amount. When a production company decides that the important episodes (i.e. pilots, whams, and finales) of a show get priority, other episodes (like filler) will, to conserve production costs, be drawn with only the bare minimum of framework that they absolutely must have.

From Executive meddling to losing key character references due to outsourcing, a lot of well-animated anime (or individual high-budget episodes of otherwise cheap shows) tend to feature looser artwork than usual in select scenes, as a way to show off the style of each individual animator and give said scenes more of a "soul," so to speak. These inconsistencies as a result of animation directors treating individual animators as artists as opposed to mere cogs in a machine is often seen as "low quality" or "lack of budget" by anime fans, who tend to value detail-per-frame and consistency above all else.

#10: YuYu Hakusho “YuYu Hakusho” had a few episodes with incredibly fluid animation at the cost of any consistency in the character models. Notably, one of those episodes was during Yusuke's battle with the Doctor, and the off-model work served to make him look incredibly like Astro Boy.

#9: Pokémon “Pokémon” has been rather obvious at this from the start, as a majority of the drawn creatures don't look exactly like the source, such as the color of their fur or their height differences (Ash's Shiny Noctowl and all of them in general are midgets compared to the real thing). Every once in a while, an episode will pop up with noticeably poorer animation, and mistakes will make it to air such as this, where Ash can be seen with two Pikachu:

#8: One Piece Or this double-vision from “One Piece”:

Because two Zoros are better than one.

#7: Mars of Destruction Ah, “Mars of Destruction.” At times, "so bad it's good," the anime infamously suffered from a low budget and therefore limited animation, as well as the practice of using mix-and-match parts in animation, rather than drawing every single new cel. The offenses made could go on and on, but are perhaps best exemplified here when one loli foot soldier gets her head blasted clean off. Not only does the blood not arc or rain down, her body slides out of frame in that very same pose.

http://www.youtube.com/watch?v=Qcn4ndi4Gcw#t=03m10s

#6: Naruto Several major battles suffer from strikingly different (though not necessarily lower quality) animation in “Naruto.” Also, most of the episodes afterwards (the Filler Arcs) suffer from poorer animation. The reason for this is that the show, at any given time, has more than a dozen studios producing the animation. Many dedicated viewers can identify different studios just by the drawing of characters. The thing is, however, not every famous instance of anime looking “wrong” is down to mistakes or rushed work. We've had several years of “Naruto” fans puzzling over this amazing sequence:

…but the episode in question, Naruto Shippūden 167, is handled by a technical director named Atsushi Wakabayashi, a storied and talented action animator who rose to prominence on the back of excellent work in the likes of “Yū Yū Hakusho,” “Ninku,” and the first “Naruto” series. In this sequence, he and his staff aren't screwing around, they're depicting Pain's primal anger in a very stylish and novel way.

#5: Ginga Densetsu Weed With the notable exeption of “Wolf's Rain,” animals tend to be done poorly and in lower quality than the rest of the art in anime, perhaps because the animators are used to drawing humans. The anime of “Ginga Densetsu Weed” is practically legendary for awkward animation/mistakes among the fandom and even has a forum topic centered around posting these so-called "bombs": http://www.gingasite.net/ginga_board/index.php?showtopic=27 (It should say something that the thing is over a hundred pages long).

One especially jarring animation mistake comes from episode 10, when Kyōshirō's mouth begins moving long before any sound is heard:

http://www.youtube.com/watch?v=U5WnFXor2iQ&t=12m28s

#4: Fist of the North Star If you truly love “Fist of the North Star,” then you know quite well that it was as low-budget as they come, with Kenshiro and company sometimes looking radically different from shot to shot. But despite quality differences, Toei's animators were generally able to make Kenshiro look scary from shot to shot-- here, though, his eyes can't seem to focus:

http://www.youtube.com/watch?v=wa1HfHd5qNU&t=1m43s

#3: Lost Universe Episode 4, “Yashigani Hofuru” (Coconut Crab Massacre) had lots of problems in its original TV broadcast version, including off-model characters, extremely slow frame rates, and ridiculously unrealistic motion (this led to "yashigani" becoming a nickname for bad animation).

When this particular episode aired, however, all hell broke loose. Fans were genuinely incredulous at the work on display, and broadcaster TV Tokyo had to post an apology. It turns out that the settei – the character model sheets, which tell the animators what the characters are supposed to look like, complete with multiple poses, costumes, and notes on scale, had been lost on the way to the South Korean in-between studio. With only a tiny stack of key frame cels for reference, the studio had to wing it, and the results were breathtaking, like this moment where they go full Game & Watch:

http://www.youtube.com/watch?v=C3Gw_L7BbEE

#2: Gundress: The Movie Another great example of “yashigani anime” is the theatrical cut of “Gundress: The Movie.” Rumored to not actually be finished, the film is full of janky, awful-looking segments making characters appear to be glitching:

http://www.youtube.com/watch?v=PL0z_pLsKKY&t=0m25s

#1: Transformers Armada Executive meddling. One of the major criticisms of “Transformers Armada.” Courtesy of Cartoon Network, the pressure from upstairs often forced animators to rush through scenes from the show leaving mistakes unchecked. These events happened not only with Armada, but also its sequel Energon, where several episodes aired in America first before the animation was even finished, and they had to drastically alter the scripts trying (and failing) to salvage the plot. It didn't help that the first episodes were animated using completely different animation models for Optimus Prime and Megatron. Energon was worse about it, though Armada infamously has a black spot on the screen with wings in the place of Starscream for a sustained period in the episode "Decisive Battle":