This can be a problem in games when, for whatever reason, a player can't make it to a session but the game continues anyway. One way to get around the absent player is to entrust their character to another player or the GM for that session. As a result, this character may end up doing or saying things they wouldn't normally do or say.
Discussions of a sort of Ultimate Showdown of Ultimate Destiny within Warhammer 40,000 will generally have at least one fan stating the abilities of each army while being portrayed in an adaptation will always depends on the adaptation. Especially the Imperial Guard, who in a Space Marine based book will just run away and die, and in a guard based book will be courageous humans in impossible war situations.
This actually happens to every faction: They kick ass in their own codex, but only appear in the other codexes to make that side look good. The 2nd Edition Tyranid Codex actually had a company of Space Marines get wiped out seventeen minutes after their Big Damn Heroes moment.
Given how vast the W40K 'verse is, both extremes (and everything in between) could coexist.
The Ultramarines have received a particularly nasty dose. They have no less than three overarching portrayals: Graham McNeill's, which portrays them as predominantly hidebound idiots who suffer a psychic fracture at the concept of disobeying the Codex Astartes, even when it's clearly not working; Mat Ward's, which portrays them as a Canon Sue who every other Space Marine Chapter wants to emulate on some level; and the rare "middle way" which portrays them as traditionalist but not suicidally stupid about it, and while they're respected no Chapter is exactly going to give up its own rites and ranks to duplicate them (previously thought extinct, it was recently sighted in Space Marine).
In the Horus Heresy novel series, which fleshes out the 40K backstory, the Emperor is portrayed as a competent and well-meaning ruler who just happens to have the parenting skills of a coffee table (Graham McNeill), a competent but vicious ruler who is willing to go to any lengths to safeguard humanity and doesn't fully comprehend that he's supposed to be a father as well as a commander, but isn't so much malicious as he is very, very ruthless (Dan Abnett), or a sadistic jerk of such magnitude that it's amazing Horus only managed to turn half the Legions, rather than leading all eighteen back to Earth (Aaron Dembski-Bowden).
On a similar subject, Perturabo suffers from such a bad case that his personality is listed as 'mercurial', presumably to try and justify why he seems so different depending on which of two writers are handling him that book. He goes between a sympathetic man pigeonholed into the part of a destroyer, cultured beneath a harsh exterior and continually overshadowed by his brothers despite his own legitimate prowess until finally he snaps from a lack of recognition (Graham McNeill) and a cold perfectionist who ordered his legion to decimate itself the second he saw it merely for not living up to his standards and who fights battles largely as a game of numbers, subscribing fully to We Have Reserves (John French)
The exact characteristics and abilities of some units or characters vary between writers, especially if the background on them has been vague in the past. For example, Pariahs tend to vary between two main types: (1) they nullify the abilities of nearly psykers and creep people out (typical of Dan Abnett's Pariahs), or (2) their very presence induces severe agony, large-scale Mind Rape, and sometimes even cranial explosions among nearby psykers. Some authors will even give them abilities seen nowhere else, such as one Pariah being completely invisibile to daemons in Fear To Tread, and water evaporating in a Culexus Assassin's presence in Nemesis.
Some of this may be explained by the fact that there are different degrees of anti-psychic ability, but only the fittingly-titled Pariah has acknowledged the discrepancies, and only in a couple of throwaway lines.
The Animus Speculum helmets worn by Culexus Assassins also either function as Amplifier Artifacts for the wearers' anti-psychic abilities, or as a means of capturing and harnessing enemy psychic attacks, depending upon which source you read (and either kind may or may not double as a Power Nullifier for the wearer's own abilities when necessary). As of the updated tabletop rules released in 2015, the dual amplifier/nullifier interpretation is correct, but with shades of the power absorption type, as the Culexus' innate abilities also now extend to stealing psykers' souls.
Tzeentch gets this a lot, owing to his status as the most abstract of the Chaos gods. Somewhat fittingly, given that he is a god of change, the writers can't seem to make up their minds about many aspects of him. Specifically, some writers believe he has a cosmic, overarching Evil Plan that all his schemes are working to achieve (and said plan is so complex, only Tzeentch himself can truly comprehend it). Others write Tzeentch as not having an overarching plan at all, and just scheming for the sake of scheming. Still others write him as a god of randomness rather than planning, where he is quite happy to institute change for change's sake, even if that change works against him.
Sometimes happens to Khorne as well. While he's well-established as being the Blood God, and has always had a tendency to be worshipped by crazed berserkers, he has also been described as a god of martial honour. This is more prevalent in older background material, and has largely been lost to Flanderization, but some authors still depict him/his servants in this way (such as in the 2012 audio drama Chosen of Khorne). It's much more prevalent in works dealing with the version from the other Warhammer setting, who may or may not be the same character.
The entire backstory of some races and even the whole universe has changed at times, especially with the introduction and later evolution of the Necrons. Initially they were little more than undead Recycled In Space with no motivation or personality other than the desire to kill, but have also been portrayed as trapped in a Faustian deal with real personalities attempting to rebel against their C'tan masters. The nature of the C'tan and their war with the Old Ones, which ultimately forms the basis of the setting, has varied hugely as well. Were the C'tan innocent SpaceWhales who became corrupted when the Necrons gave them bodies in an attempt to harness their power, or did they deliberately corrupt the Necrons and steal their souls? Are they vulnerable to psychic powers with no connection to the warp themselves (originally how the Old Ones were able to beat them), or is one of the C'tan actually the Chaos god Tzeentch?
Genestealers have also changed quite a bit from their early incarnations. Initially an army in their own right, they became first an infiltration force for the Tyranids that served as a vanguard for hive fleet invasions, in the form of Genestealer Covens/Cults, and then became a standard assault troop for Tyranids with no variations of unit types and no army list of their own (Genestealer Cults remained a part of the lore, although they were often overlooked). In 2016, this swung back the other way when Genestealer Cults made their return in a Gaiden Game, and then received a full rule and model release for Warhammer 40,000 proper soon after.
Orks are usually not meant to have any connection to the warp, instead generating their own power simply by proximity to other Orks. But some writers have described them interacting with the warp and attracting demons in exactly the same way as human psykers. Their technology suffers similar issues, sometimes being crude but functional equipment built by instinct, and entirely usable by humans if the need arises. Other times it works entirely by Clap Your Hands If You Believe, with a box full of loose scrap metal able to work as a powerful gun as long as the Ork thinks it will.
Exactly how ignorant most Imperial citizens are of Chaos varies between authors. Sometimes they may be familiar with legends or rumours about the Horus Heresy, the Warp, or the Traitor Legions and their Primarchs, while at others, they are not only ignorant of Chaos, but also of other, relatively mundane threats like aliens (despite "abhor the alien" being well-established as a central tenet of the Imperial Cult in 40K lore). Even high-ranking Space Marines can vary from knowing almost nothing about Chaos beyond its existence (enforced by mind-wiping any Marine who comes into contact with daemons in some cases), to having a working, if basic, knowledge of the Chaos Gods and their main types of daemons.
Warhammer is written by dozens of different writers so this happens a lot. Even down to how entire cultures within the setting act.
White Wolf tends to have a few problems with this, and one stand-out example is in Changeling: The Dreaming, where the writers kept going back and forth on what "Banality" was, aside from "the death of hope." Banality was trying to define and tie down the world too much... except the nockers kept insisting that the moon landing resulted in the biggest rush of glamour anyone had seen in several life times. So Banality was boring, ultra-focused practicalities... except there were sample NPCs who got Glamour from those activities because of mindset. When it got to the point that LARP was associated with the Autumn People, you knew there was a basic communication breakdown. Sadly, the line was cancelled before The Book of Glamour (which would have laid out some basics on Glamour and Banality) could be released.