Parting thoughts...
On why people make up rumours:
There's cache that comes with being an "insider". It's a way to set yourself apart from the pack, because only you are cool enough to have access to this information. Furthermore, if you can invent rumours that are exactly what fandom want to hear, they'll be so psyched at the news that they'll praise you for - essentially - reassuring them that everything is awesome. This isn't a Games Workshop phenomenon; go to any decently-sized forum discussing an upcoming movie / TV show season / video game / novel and there's going to be at least one bullshit artist revealing fraudulent "leaks" and bathing in the attention and status it gets them. Knowledge is power, especially when dealing with obsessive geeks, and when there's no way of testing that knowledge until release day, creating the illusion is simple.
Whether this is the genuine motivation behind people like Stickmonkey, TheDarkGeneral and Ghost21 consistently lying about upcoming releases isn't something we can know for certain, since we aren't them. It is, however, the simplest explanation, so we may as well stick with it until something else comes to light. If it turns out that the Jigsaw Killer has kidnapped Stickmonkey and forced him to lie about upcoming the Dark Angels codex in order to disarm the bear trap around his neck, then I apologise. He is, however, still lying.
On guesswork and confirmation bias:
You'd think the fandom of a game based on probability would be harder to fool when it comes to passing off educated guesses as facts, but apparently not. So, let's explain this in 40k terms: Orks have Ballistic Skill 2, meaning roughly 1/3 of their ranged attacks hit the target. Get a big enough unit of Orks together and, with luck, you can rack up a fair few hits in the shooting phase. It would be ridiculous, however, to use those hits as evidence that Orks are good shots - it's clear that the hits were the result of a high volume of fire, not the skill of the Orks, and that most of the shots still missed their target anyway. So, when Stickmonkey drops 262 rumours and only 74 turn out to have any substance - A hit rate of 28%, which is even worse than the Orks - there's every reason to call bullshit. When TheDarkGeneral manages to land 12 hits off of 89 rumours (18%! EIGHTEEN PERCENT!), then anyone who keeps listening to him is being played for a sucker.
It's also worth looking at the "hits" these people are getting. Things like "Plastic Plaguebearers" come true because - well, why wouldn't they? Plaguebearers didn't have a plastic kit at the time, and, as a core / troops choice for two armies, it's a no-brainer that they'll get a plastic kit. The same goes for such groundbreaking revelations as "Plastic Deathwing box", "Grey Knights get an Inquisition character", "Metal characters will be finecast" and so on. I'm certain that Stickmonkey's rumoured Tzeentch daemon chariot pulled by Screamers comes out one day, too: I don't have any info whatsoever, I just know that it's a unit with no kit, and that any Tzeentch thing logically has to be pulled by Screamers (What else? Horrors? Flamers? Ahriman?), so therefore it is, at the very least, on Games Workshop's to-do list. The same is true of his "rumour" that Forge World are going to release a Horus model for their series of campaign books based around... Horus. Woah.
New units can also be predicted by simply looking at Games Workshop's business practices: There's a current push toward larger kits, whether it's 100mm x 50mm monsters in Fantasy or large vehicles / monstrous creatures in 40k. It's therefore safe to assume that any army not in possession of these things will gain access to them in a new update - and, lo and behold, we get vague reports of a "mystery large vehicle" for Dark Angels and an "uber-suit" for Tau. The same is true of flyers in 40k. The flyer mechanic is new, not all armies have flyers, therefore Games Workshop are going to start doing flyer releases outside of the normal codex schedule. We're so far missing the unknown Eldar and Tau flyers, as well as the Tyranid Harpy. I expect them out soonish. I have no source, insider info, leaked documents or "playtest rules" to back this up, simply the fact that they represent a hole that Games Workshop are currently working on filling. I also expect updated Harpy rules, because Games Workshop must surely realise how badly they fucked up the Tyranids by now.
My personal favourite of these is Stickmonkey's exclusive, super-secret insider report that - are you sitting down? - the Vindicator would be "unchanged" in the Dark Angels codex. This was reported as a rumour, even though it literally tells us nothing.
On vagueness:
Words and phrases to look out for in rumours: "Mystery vehicle", "Has an additional effect", "Supposedly very powerful", "Looks sick", "Expect something big"... you get the drift. The only circumstance in which people talk like this is if they're trying to get people interested without letting any actual information slip out. I can fully understand the actual game developers talking like this publicly, in con appearances or interviews or at the pub or whatever, because they want to get fans excited without violating their NDA's and/or gentlemen's agreements.
An anonymous "source" leaking information is already violating their contract and relying on their anonymity to protect them - there's no need for them to use vagueness, misdirection and open-ended statements, especially ones that sound like marketing talk. Someone who had actually seen the models and/or read the accompanying rules would be able to give a concrete account of them, or at the very least something more descriptive than "It's very powerful" or "It looks sick". This is crap. Ignore it.
On lies, damn lies, and playtest rumours:
The army books and codices are not still being tinkered with two months from launch. If - as seems likely - the next big release is Daemons, and it's scheduled for March, then their book is already finished, the playtesting phase is long over and the designers are working on something else. Anyone who claims to have a source leaking them "playtest rules" for the Daemons book(s) right now is talking shit. The same goes for when we find out what's due in April and miraculously start getting leaks from that particular round of playtesting, etc. etc.
Ah, but: Some people, like TheDarkGeneral, have given us "playtest rules" well, well in advance of the actual releases. Were they accurate? Fuck no, they were completely off the planet: Alpha Legion controlling enemy vehicles, units of Berserkers on Juggernauts, a new Land Raider pattern, army-wide special rules for each legion, Chaos Scouts, vehicles with sonic weapons, daemon engines operating in packs... all absolutely nothing like the book we actually got.
And that's the key thing: It was absolutely nothing like what we got. That this was all "playtested" simply isn't believable in light of the finished codex, because there's no correspondence between the two sets of rules. To go from the rules-heavy, legion-driven codex packed with never-before-seen units and characters to the (relatively) streamlined, single-list-that-does-everything codex with a few new toys in it would involve a radical rewrite of the whole thing from the ground up, not going over playtest feedback. It's bullshit, as usual.
On how things are actually leaked, 95% of the time:
Someone gets hold of a copy of the rules about a month in advance. That's about it.
Games Workshop have proven very, very adept at keeping schtum over the last few years, and things only get out when they want them to. Yes, there are exceptions, but not many. Precisely why they don't want anyone knowing what's on the horizon more than a week in advance, I have no idea. It may be that they want their customers' undivided attention given over to what's out now, as opposed to what's coming out later in the year, because that encourages them to spend rather than save - but that's pure speculation on my part.
On why this is important:
So, here's the thing: Why do I care? Shouldn't I "just enjoy the speculation", as is the refrain? Well...
1) It's hard to "enjoy the speculation" when anything, no matter how vague, useless or obviously fabricated, is uncritically presented as equally valid "news". Once it becomes obvious that a website not only won't question the validity of the information it passes on, but will actually delete any comments from members that do, it becomes untenable as a news source. The end result of this total lack of quality control (except when it comes to people who criticise the lack of quality control, of course - they're filtered out almost immediately) is that each piece of genuine information about an upcoming release is buried under a mountain of shit from people who have - in at least one case - been wrong literally hundreds of times. What this says to me, and probably to most other people, is that you care more about how many people are reading you rather than whether what they're reading is accurate, informative or useful, and you've become the tabletop gaming equivalent of a trash tabloid that publishes fake celebrity gossip and/or UFO abduction stories because they're an easy way of reeling in readers. In which case, fuck you too.
2) It devalues legitimate rumours. If you were a bystander during TheDarkGeneral's enchanted carnival of bullshit around this time last year, you may recall what happened when Blood of Kittens first leaked Chaos Space Marine rumours that were actually, you know, real. People hated them. People were angry at BoK for posting them. People made a point of ignoring them, in the face of all evidence, because TheDarkGeneral's stuff just sounded so much cooler, and considered "It's basically like the last codex, only it isn't shit and there's a robot dragon" to be a let-down. There's been some moaning about how DakkaDakka's rumour accuracy tracking will scare off people who are worried about being labelled frauds, but surely the current environment scares off people who have legitimate information that doesn't say exactly what people want to hear?
3) It affects how people think ahead. No, I don't have hard data on this, and I don't know how one would obtain it, but "knowing" what's on the Games Workshop horizon is going to affect people's plans. I'm sure someone, somewhere, actually converted up some Brazen Knights upon them being "confirmed" by Ghost21 and TheDarkGeneral, only to discover they weren't real. Or maybe they put off working on the project they really wanted to, on the grounds that they needed to save up for the new Harpy kit that didn't come out. Maybe they bought a bunch of Tau after being told, over and over, that they were the next army in the pipeline. Or they passed on some metal Sister of Battle after being promised that plastic ones were coming out by the end of the year. Now, of course, the fault is ultimately that person's for making a decision based on unconfirmed rumours, but the fact remains that it wouldn't have happened had the false information not been taken seriously and all criticism of it deleted on the grounds that it's "harassment".
On the actual reason I dislike Natfka:
Nothing to do with policing comments on Stickmonkey, strangely; it's that he thinks this awful thing called "Codex: Women" is funny and reblogged it. No, I'm not linking to it. Google it. It's shockingly sexist and confirms every single negative stereotype about gaming being a hobby for snickering adolescent boys. His blog has like four million hits, so it's not like we can pretend it didn't happen.
On why I keep mentioning Stickmonkey and TheDarkGeneral by name:
You can't do it anywhere else and not get your post deleted. They're both liars who routinely mislead people and deserve to be shunned for it. End of. I'm sure they'll retire those pseudonyms eventually and the whole process will begin again, but hopefully the community will have learned by then.
On the DakkaDakka accuracy thread:
Yes, of course it's a good idea. Amazingly, there's a whole lot of Stickmonkey nonsense that isn't covered there. Go to Natfka and search "Stickmonkey", if you want a laugh.
On the sudden leap in quality of Warseer's forums now that Stickmonkey has stopped posting:
Nice, innit?
All stats in this post gleaned from the DakkaDakka accuracy thread, and correct as of Jan 31st 2013.
I still got more Dark Angels stuff right than most people, y'know.