It's actually fairly common for comic books to stray from the model. Even in the same line of comics, one artist's style might vary from others', and some might draw the same character with different proportions. For example, someone might draw Superman as an enormous bodybuilder, where others might draw him with a build more akin to Christopher Reeve.
Artists Randy Elliott and Stuart Sayger received much criticism for their work on LEGO's BIONICLE comics. Randy notoriously drew humongous shoulders on many of his characters, sometimes gave them very small and thin heads, and in the case of two of the Toa Metru gave them obscuring visors instead of bothering to draw their eyes (which weren't even the right colours). Sayger, on the other hand, practiced a unique, sketchy style which — while probably very serviceable in any other work — simply did not work well with the types of intricate character designs present in BIONICLE. This was especially obvious in "All Our Sins Remembered," which presented a jarring contrast between the beautifully rendered element lords and his characteristically stringy, weirdly proportioned designs for the other characters. He proportioned the characters in so many different ways that a whole Off Model gallery could be filled with them. And in one instance, he literally used the wrong model for a certain character — instead of Mata Nui's correct, originally planet-sized, bulky and blocky robot body, he drew the mortal form that he took on one hundred thousand years later, which (apart from also being a humanoid) looked absolutely nothing like his former body.
There's also a panel in the last 2008 comic (drawn by a different artist, Leigh Gallagher, whose work is far more well-regarded) in which Turaga Dume is drawn based on his design from the second movie, whereas everyone else is modeled after their toys. This would have been acceptable (because it was really well-drawn) if Elliott hadn't drawn Dume based on his toy back in 2004 (incidentally, that too was also off-model in that it wasn't even the right colour).
Colors have always represented a rich source for off-model moments, going back to the very first issue. The random color accents which weren't there on the toys themselves varied from panel to panel, and often whole bodies appeared sporting the wrong colors (fully dark green Nui-Rama, striking red Lewa, and perhaps most infamously, green Nokama and blue Matau). There were instances when the glowing effect used for the eyes were placed to the wrong spot, although granted, this mostly happened to beast characters, which tended to have eyes that didn't really look the part.
Even Carlos D'Anda, the best-regarded comic artist of the series had his moments - the entire last page of the very first comic issue has Kopaka appearing with a minuscule head and Pohatu sporting three radically different designs in separate panels. In one, he has the wrong arm-type and his torso is right side up (he really has an upside down torso).
Rob Liefeld's artwork is frequently, and rightfully, accused of this, as his highly stylized art often deviates from the model sheets used by other artists.
He famously couldn't keep Cable's eye glows consistent from issue to issue. He created Cable. Shatterstar's knife also kept switching from one blade to two.
Pat Lee's work on the Transformers comics when Dreamwave held the license was horrible about this, with everyone in constant Dull Surprise expressions and proportioned inconsistently. It only gets worse when you also include his covers for this series. Combine that with him using ghost artists and you get an artist that's not only worse than Liefeld, but also lazy and greedynote apparently, much of the funds that should have went to the other artists instead went into the company's Porsche.
It should be pointed out Lee was "discovered" BY Liefeld at a convention and brought in as an intern. The rest is poorly-drawn history.
Before this however, Transformers Generation 2 and the original comic series exhibited this. The original had several artists and thus different styles for eachnote the early issues also featured several early and/or unused models for a few characters, like Megatron. G2 on the other hand only had three artists - Geoff Senior, Derek Yaniger and Manny Galan. Yaniger drew everything Liefeld-like - proportions all over the place, giant guns and pouches everywhere, sneers-aplenty and Schedule Slip all over. Manny Galan tried to ape Yaniger's style to little success, while Senior stuck to his chops but carried over Yaniger's heavy arsenal.
During his run on the first Transformers series, Jose Delbo was determined to stick to the character models, so much so that he inadvertently made Starscream particularly off-model - by dropping the "vents" from the right side of Starscream's head (in the model sheet, it was obscured by his shoulder fin). Subsequent artists restored Starscream's right vent-"ear".
The Beast Within's artwork is nothing short of a horrendous failed attempt to ape Pat Lee's style. The second issue is slightly better with the artist working in a style he's more accustomed towards, but the art still falls short.
In a weird example overlapping with Depending on the Artist, the IDW comics had a problem where, thanks to lax editing, character designs would change from one issue to the next if the artist changed. This lead to weird things like Blurr driving away in Earth alt mode only for the issue following up that scene drawing him in a different design. Things smoothed out after John Barber became editor, but it still crops up, one issue of RID depicted Hardhead as a bunch of rounded shapes that looked nothing like his actual character design.
What exactly is Carlie Cooper's character design supposed to look like? To hell if the rotating staff of Spider-Man artists know! This led to a cringeworthy and hilarious scene where she was preparing to sleep with Peter and look scarily like Mary Jane (Carlie is Mary Jane's Replacement Scrappy as main love interest in the book). A lesser example is how colorists couldn't decide how dark Lilly Hollister's skin tone was supposed to be in her intial arc.
On some pages of issue 630 of The Amazing Spider-Man, artist Chris Bachalo draws amputee Curt Connors with two arms.
At one point in volume 9 of The Walking Dead, Rick's hand somehow magically grows back.
Poor, poor Bart Allen. After he became Kid Flash, a lot of people started coloring his hair red like he was Wally, despite one of his main physical traits being that huge poofy brown practically-a-non-curly-afro on his head.
While the art was much more consistent compared to the anime, these◊ two◊ examples of the You're Under Arrest! Manganote mentioned here as it waspartially released in comic book form in America stand outnote the watch present in the first example only appears in that one panel.
For a time, Sonic the Hedgehog took Depending on the Artist to such extremes that there's a rumor going around that until recently the comic's quality control was turned off to speed up production. While nothing has been confirmed, with panels like this going all the way from initial pencil work to ink to coloring to print apparently without anyone noticing its sheer (and hilarious) horribleness, one has to wonder.
A major case of Off-Modeling came from the change of Sonic's appearance from his classic look to his modern, green-eyed look. While it boiled down to just Sonic with slightly longer quills, the artists had no idea what he looked like and would give him wildly different quill designs where it would look like he had REALLY long quills.
Then there was that time where the colorist apparently mistook Sonic's mother for Amy...
Often done intentionally with the length of Batman's cape. Under normal circumstances, it's just long enough to reach the floor when wrapped around him like a cloak for his famous "looking like a scary gargoyle" pose. Other times the artists can't resist having the wind dramatically blow it out, making it look like it should drag several feet behind him. At least one official DC Comics "How to Draw" book instructs people on how to do this without it looking too absurd.
In the Batman issue when Stephanie Brown is thought to be dead and her funeral is being held, both Bruce Wayne and Tim Drake are very off model. Bruce looks like a tall skinny version of his The Batman counterpart and Tim looks like he's about 8 years old; he's supposed 15 or 16 at this point.
At one point in Captain America Return of The Asthma Monster, after defeating a snake creature, Captain America scratches his chin and gains a clawed hand for some reason.
The micro series in general suffers from beautifully-drawn pencils and ink suddenly becoming a lot rougher and more awkward-looking during the coloring process. The main series generally leans towards more consistent artwork.
Similarly, some of the variant covers (drawn by artists who don't usually draw ponies) can seem a bit off compared to the show's original designs. For example, this.
Humberto Ramos' run on Runaways started out with dodgy art (his renditions of Nico Minoru were especially hideous, to the point of having Unfortunate Implications), but got somewhat better. YMMV on whether or not the artists who replaced him were necessarily better.
Novas Aventuras De Megaman (not to be confused with the Archie comics) was really bad about this. You see, unlike most comics, the writers would make a script, then have fan artists draw it. The one they felt fit the story best would have their work picked. Unfortunately, this meant that few, if any details from the previous issue would be carried over into the next. The only artist to draw more than one issue was Rogerio Hanata.
Archie's series has its own moments of Off-Modelness, though thankfully, it isn't half as bad as the Brazilian comic's problems. It all boils down to Chad Thomas and (to a lesser degree) Ryan Jampole using a different style from other artists.
In the issue of X-Men Annual that tied into Atlantis Attacks, the artist drew Wolverine as your typical 6'3", 8.5 heads tall superhero. Other than that, the art was fine.
In the first issue, Nightwing is wearing a completely different costume to the one he was currently wearing in his on-going series, specifically the costume he hasn't worn since before Death Of The Family.
Also from the first issue, in the single panel Kid Flash appears in, he manages to have short blonde hair instead of brown Anime Hair; and the Flash's Chest Insigniaon his chest when he not only doesn't have the logo there, but the version on his costume is flipped.
There are a couple in the Crowded Cast Shot: Harley Quinn has pink skin instead of chalk white, the Trickster's right arm is normal instead of an Artificial Limb, and Grodd looks more like a Sasquatch than a gorilla.
Black Manta is shown without his notable facial scars.
Wendy Pini of ElfQuest once managed to stray subtly from her own model by giving one of the elves a five-fingered hand instead of the usual four.
Older Than They Think: this cartoon from an 1884 issue of Puck, in which James Blaine is depicted as having abnormally large buttocks. This is inconsistent even with artist Bernard Gilliamís other depictions of Blaine.
In Deadman and the Flying Graysons, set in the Flashpoint universe, there's a confusing one. There's a panel where Dick Grayson and his dad seemingly swap costumes, as the Grayson in red calls the other Grayson "dad". Before and after this panel, the Grayson in red is Dick's dad.
Back when Huey, Dewey and Louie all wore red caps, sometimes a fourth nephew would be accidentally drawn in a panel.
In print editions of some early issues of Avengers Arena, X-23 was drawn with three claws in each hand, in the same arrangement as Wolverine. Digital editions did fix this, and give her claws in the proper arrangement of two in each hand.
To a lesser extent, Laura's canonically green eyes were frequently colored brown in Avengers Academy, while ever since joining the cast of All-New X-Men they've been consistently colored icy blue.