The Greyhawk examples are two weak examples.
The first was a deliberate change. Canon wasn't ignored, the world was just altered.
There are two changes to canon in the transfer from folio/original GH boxed set to FtA.
First, there is the disregard of the invasion of the Shield Lands by the Horned Society that is a factor in the City of GH boxed set. Yes, it can be retconned into the FtA timeline, but it's not there by default and not a natural fit.
Second, and perhaps more importantly, what a lot of people liked about GH was it's non "one big evil" theme, and its more pulpy/S&S feel - which FtA undoes. This is analogous to your example, upthread, of a writer disregarding the personality of an established character. In this case it was a change in the moral/dramatic tone of the gameworld.
If you were writing a game supplement "Templars of Tyr" I'd assume you were doing the research and compiling all the past references. Small things (colour of attire) don't matter much, but there are big details that can shift.
When writers are given free reign to ignore continuity and canon, everything goes out the window as people try to leave their mark or rewrite small pet peeves or self declared bad ideas.
You can kinda see this modern comics, where every writer dramatically changes characters and they need to reboot and re-reboot comic lines every five years
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D&D lore is a little more flexible in terms of character, but the backstory of monsters and the universe shouldn't change on a whim. The entire backstory regarding giants, or mind flayers, or elves shouldn't just change in the base game. That should be consistent and shouldn't be reworked every time some writer has a new idea they think is cool.
I like Canon and there is nothing worse then other people derping things up.
That's the thing about "canon." If you're too slavishly devoted to it--and I mean this from the perspective of a writer/content producer, not just a gamer/reader--then you miss out on opportunities. Sometimes, a later idea is better than one that's already been accepted.
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D&D? Is a series of pick-and-choose ideas for you to build your own games first, and a platform for prewritten stories second. So no, I don't have a problem at all with things like the conflict between the Illithiad and LoM.
On this issue I absolutely agree with Mouseferatu - and that is as purchaser/player, not as an author.
I don't read D&D setting material, or MM lore, or whatever, to "learn about the D&D world" (whatever that is). As Mouseferatu says, I read it to find stuff to pick and choose from to build my game. So I want authors to give me their best work. If that means changing something, well that's fine - I'll sort that out. If I don't like what the author has written, it's no harder to ignore something they say about the past of the setting, or the geography of the setting, then it is to ignore something they say about the future of the setting, or the personality of a NPC.
If book 1 says the origin of giants is X, and book 2 says that it is Y, then I can choose whichever one I like. (If it's a small thing, there's the risk I'll forget which one I chose - but then if it's a small thing it probably doesn't matter if I forget! And if it's a big thing, then I'll remember, or it will come out in play and my players will remind me.)
Btw, as far as I understand the reason for frequent reboots in comics is precisely to kill off all the canon, so that new fans can get into the comics without having to grapple with accumulated lore that is, from their point of view, largely pointless. (And it's worth noting that plenty of comics used to functionally reboot back in the day as well - every few years there's another "supervillain tries to marry Aunt May" or "Peter breaks up with MJ" or "X-Men get taken prisoner by Magneto" or whatever story.)
What other errors are there? Will this contradict something that has already been established in my game? You just don't know...
To me this is hard to work through.
I mean, suppose that in my GH game the PCs kill Nerof Gasgal, the mayor of GH. And then I buy a new GH supplement and it assumes Nerof Gasgal is still alive. That contradicts something already established in my game. So I have to disregard some of the supplement.
Conversely - suppose someone publishes a GH supplement that rewrites who is the mayor of GH. It's not Nerof Gasgal any more. But in my game, following the older material, it is. So I have to disregard some of the supplement. The "error" on the part of the new author is no bigger a deal, in this case, than the "canon continuity" of the author in the first example.
As Mouseferatu says, it's all just stuff to pick and choose from in playing a game.
Compare this to... oh, the new cosmology of 4e.
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It was needless. The World Axis could have been presented as the background for the Nentire Vale setting.
It could have been, but how would that have made it better? I have used the core cosmology and history of 4e (ie the gods; the planar set-up; the fallen empires of Bael Turath, Akhosia and Nerath; etc) but have zero interest in the Nentir Vale (the earthly part of my 4e game takes place on the map on the inside cover of Night's Dark Terror, so is notionally the Grand Duchy of Karameikos - though I believe the Grand Duchy may not actually have been mentioned ever during play, except perhaps in the first session).
So for me it was better to have the cosmology and core conceits presented up front, and to have the Nentir Vale relegated to a chapter of the DMG that I didn't read for the first few years that I owned the book.
And the cosmology is novel and interesting, whereas the geography and details of the Nentir Vale add nothing very novel to the abundant supply of low-level adventuring settings (hence the fact that I can use Night's Dark Terror instead and need make no other changes to my default lore 4e game).
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