What does it matter if I change something by ignorance? Eg in my recent Dark Sun sessions, I've described the templars of Tyr being dressed in red. I have no idea what the canon is for this, but I needed something in the course of play to describe an NPC; and red seemed a good colour for sinister templars in a desert city.
If you're running a game, it doesn't matter.
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. (Unless it was a major plot point, like how templars of Tyr wear blue because those of another city state wear red. With blue being the colour of the city-state of Tyr...)
A bigger change would be something like the origin of the Templar order, common behavior, or some goals. If suddenly the Templar were formed at a very different time or are doing very different activities than established in past products, it creates an annoying contradiction.
And as for changes by the author/publisher: if I prefer (say) folio GH to From the Ashes GH, then why can't I just use the version that I prefer? Or another GH example: the City of GH boxed set has Shield Land refugees in GH, and the later From the Ashes GH mostly ignores that. I don't know if that was deliberate or careless on the part of the later authors, but why would I care? I just use the version that I like (in my case, I've used those refugees, and the Horned Society war on the Shield Land, in two different campaigns).
The Greyhawk examples are two weak examples.
The first was a deliberate change. Canon wasn't ignored, the world was just altered. Which is an entirely different issue. It wasn't a retcon: the changes were (mostly) explained. There was a continuity of events between the folio and
From the Ashes. Really, it meant there was *more* canon.
And because there was a change in era, you could ignore or not.
The second is largely an omission. It's not mentioned in the second, but I doubt it says there are not Shield Land refugees there or that there never were. The one does not cancel or change the other.
Compare this to... oh, the new cosmology of 4e. Because it's a convenient example.
There was not transitional event between 3e and 4e (save for the Realms, and that was localized). And yet the entire planar structure changed. For everyone. Suddenly there were primordials, the Dawn War, angels could be good or evil, outer planes were planets in the Astral Sea, etc. Why? Because someone liked that cosmology better and wanted a better backstory for the universe, despite the ones that already existed. Which I don't approve of.
Now, I
like much of the World Axis cosmology: the Feywild, the Shadowfell, the Elemental Chaos, primordials, etc. But making it just mandatory and assumed is disrespectful for what came before.
It was needless. The World Axis could have been presented as the background for the Nentire Vale setting. Primordials could have been added into existing giant lore. Titans could have remained grecco-roman with primordial titans being a new addition. Authors writing for an established brand or property should BUILD on what was already written, not knock things down and start from scratch. The writers of D&D are not its owners, they're just stewards of the brand and should be respectful for what came before.
For another example, the 3.5e book
Lords of Madness, gives an entirely new origin for Mind Flayers, completely contradicting the 2e book
Monstrous Arcana: The Illithiad in numerous ways. And at the same time created a contradiction that didn't mesh with the origin of the two Gith races.
What that tells me is that someone either didn't do their homework, or thought their ideas were so cool they didn't have to
try to reconcile them with what came before.
A book like
Lords of Madness should have been comprehensive: the one-stop-shop for all illithid lore, compiling tidbits from a myriad different sources. You buy it to reap the benefits of the author's work and research compiling existing illithid lore. Instead, the author didn't do the research and now you have to do the work of reconciling two pieces of incompatible lore.
These are extreme examples of the issue, but the emphasis the point.
Small errors in canon are easier to ignore, but getting the lore outright wrong or just inventing crap rather than looking it up means someone
just didn't do the work. Which means everything else mentioned could also be contradictory. And if you're looking at a sourcebook with a glaring continuity error, it casts doubt on the rest of the book. What other errors are there? Will this contradict something that has already been established in my game? You just don't know...
