4. The Von Restorff Effect
Simply put, something that is unusual is more likely to be remembered. Another popular Diablo myth involves "double drops" - getting the exact same item drop twice in rapid succession. This myth often also encompasses when someone in your party gets the same drop you received shortly after you do - because we need to stretch the definition in order to find enough examples of this happening to perpetuate the myth.
Theories about server lag causing some form of rollback glitch abound, and this myth survives on the hope that someone can find a way to control this effect - such as with the Diablo 2 duping exploits. But we'll discuss the illusion of control later...
Here's the thing: how often do double drops happen? We believe they happen more often than they do, because we are discounting the hundreds of times we don't get the same item back-to-back. The odds of the same item dropping twice in a row are exactly the same as any two other specific items dropping one after the other. On two coin flips, you're just as likely to get heads twice in a row as you are to get a head followed by a tail. But getting the same item twice in a row appears unusual to us, so we remember it better, granting it more "availability."
5. The Availability Cascade
This self-reinforcing cycle leads a community to adopt a certain collective belief. As a new idea enters a community and rises in popularity, a feedback loop is caused: because it is rising in popularity, more people seek to adopt it, and so it continues to rise in popularity.
We often see this in the form of insight or advice gleaned from top-end or professional players across any game. People watch the top-end players to learn how to become better and pass along what they believe to be advice to other average and new players. The problem is that these observations are being made in a vacuum, without regard for the numerous other factors that come into play.
The assumption being made is that if you take tactic X and apply it to an average player, it will have the same affect as on a top-end player. That advice may be beneficial to a pro player, but detrimental to an average player that lacks certain skill sets, knowledge, or gear. On the surface, though, the advice seems solid, especially when it is being demonstrated by an expert, and so the advice propagates throughout a community.
During my years in a StarCraft clan, I would see average players pass along advice to new players all the time. Often, this advice would only benefit advanced players. New players - who should have been working on mastering the fundamentals of the game - were instead learning to execute advanced tactics, and couldn't understand why they weren't winning more games.
This leads to...
6. The Illusion of Truth Effect
A person is more likely to believe a statement that seems familiar to them than one that does not. Another way of putting this is: say something often enough, and it will become the truth.
If you keep reading on forums that X is a good tactic, eventually, you start to believe it and repeat it as well. Maybe it actually is a good tactic - but the point is you haven't critically thought about it yourself. You just assume it is true because you keep hearing it.
Of course, this ties into...