Over-reliance on spell-checkers is part of it, I think. The majority of errors I find are the sort of things that a spell-checker would never have noticed, like where / were or rogue / rouge or wood / would or, my pet peeve, wreck / wreak. *Wreak* havoc! Not wreck havoc! Argh!
I've actually done some proofreading for White Wolf, and edited one (very small) book for them, only to groan when a thread popped up about the ridiculous amount of errors in the book. (The map was backwards or something during the writing, it seems, as just about every reference to direction in the book was wrong. If the text said 'we went east to X' then country X was inevitably to the west. I consoled myself that I hadn't actually ever seen the map during the editing, so I had no way of knowing that, but it still rankled to be reading a thread about how much I suck...)
It was educational to see what sort of 'errors' crept in to the document. A cut-n-paste error had every single sentence ending with the letter 'r' having the word 'river' appended to it. One of the two main narrating NPCs changed names, and race, halfway through the product, but since the chapters had all been jumbled into a new order during a last-minute design change, it seemed like she switched names every other chapter! Good times...
Lots of 'flavor text' in D&D has mechanical meanings. If a population is said to have lots of 'sorcerers,' or a 'gift for sorcery,' but actually consists of *wizards,* that's actually an issue, as a Sorcerer is not the same things as a Wizard. The terms aren't interchangeable, and if you want to refer to both, it's easier to just say something like 'arcanists.' These days, other 'flavorful' descriptors for arcane spell-casters, like Magus or Warlock, might also refer to specific classes or prestige classes, adding to the layers of confusion.
As for stat blocks, John Cooper needs to be given every book with stat blocks before they go to the printer, a wad of cash and a red pen.
About the only time errors *really* piss me off is when I purchase a product and find out that the contents have changed since the cover blurb was written. My copy of the Jungles of Chult promises an 'all new system of magic, based on gems!' on the back, and inside, no such system exists, only exiting new rules for half-naked jungle dwarves.
And don't even get me started on the old Menzoberranzan Boxed Set. I listed 200 errors and mailed the list to TSR. It was a slow afternoon at work...