1) "Lawful Good is more good than Nuetral Good"
This, and its counter part "Chaotic Evil is more evil than Nuetral Evil" is the most common falacy in the game. Lawful good is the philosophy that seeks two goals, good and order. Invariably it must sacrifice some devotion to good to maintain its devotion to order. Nuetral good is the most good because it always has goodness as its highest goal.
What is true is that lawful good people believe that they are more good than nuetral good ones.
2) Everyone knows their own alignment
This myth is subtle because its almost never stated explicitly. But in fact, it would be bizarre if everyone knew their own alignment. What's more likely is that a large majority of all people believe that they are good and describe thier own beliefs as good, and sincerely believe that what they believe is true goodness. A lawful neutral sincerely believes himself to be a good person and knows he is a good person because he has honor, obeys the law, dispenses justice, respects his betters and deals fairly with his inferiors, opposes disorder, promotes the general weal and strength of society, and otherwise does all the things which to him are the sum total of what being good is all about. Anyone that doesn't do these things, must not be 'good' regardless what they say about themselves and in fact anyone who does the opposite (chaotic neutral) must in fact be evil.
In other words, a decent sized chunk of people will always orient the alignment chart so that they are at the top. Conversely, many people who doubt their own goodness, will because they are sincerely good and humble and self-critical and percieve thier own failings (and knowing what true righteousness is magnify these failing in thier own eyes) will believe that they are in fact not worthy of being called 'good'.
This is in my opinion largely what is responcible for the depth of confusion regarding what the different alignments mean. Different people will have different definitions based on thier own personal beliefs. For example, a lawful neutral might well - when describing lawful good or nuetral good - ascribe to those alignments the beliefs of lawful neutral, and when describing neutral evil ascribe to that the beliefs and traits of chaotic neutral. In other words, it is difficult to be objective when describing moral systems.
3) "I play evil characters because evil means you can do whatever you want."
This is sort of the opposite tack of #2. People with this take on the alignment system believe that good is stiltifying and rigid and oppressive because they associate the order with goodness. Naturally, they percieve that evil must be the opposite - liberating, self-empowering, and so forth.
In fact, evil characters are just as 'shackled' to thier convictions as good characters and if anything more so. Evil is good's antithesis, not merely its opposite. Just as good characters cannot choose to not do good when it suits them and long remain good, evil characters cannot choose to not do evil when it suits them and long remain evil. Evil means doing evil even when it doesn't make sense to do evil. If you don't do evil because its in your self-interest to not do evil, then you are not really any different than the one that doesn't do good because its not in thier self-interest to do good. If self-interest is your motivation, you are much more likely to be chaotic neutral than neutral evil. That doesn't mean an evil character can't do good intentionally or unintentionally, but no more often than a good person can do evil. Which brings us to the next most common myth.
4) People of an alignment always follow the dictates of thier alignment.
The only people for which that makes any sense are those whose philosophy is neutral apathy and who don't have any dictates to follow. For everyone else, they are going to stray and fail and otherwise not resemble a paragon of what they claim to believe. Only people with very high wisdom scores will have the discernment and will to not only know what is 'right' according to what they believe, but knowing what is 'right' to act on that conviction.
5) "True Nuetral is the rarest of all alignments"
This is actually from the 1st edition PH, so it counts as one of EGG's myths. It might be true if the only sort of True Nuetral was the aesthetic highly philosophical kind that believes in balance, harmony, and moderation as the result of a particular intellectual process. But the vast majority of neutrals don't have an intellectual conviction. Instead they arrive at thier beliefs through a sort of philosophical apathy. They don't care, or they don't think about it, or they are too busy surviving, living, or enjoying themselves to worry about it. They aren't particularly trying to be anything, and so they fall on the middle of the bell curve by accident or default more than design. 'Everything in moderation' is very much an everyman definition of 'good', even if they themselves don't realize it. I'd be terribly surprised if True Nuetral wasn't in fact the most common alignment among humans. EGG's 'seekers of balance' represent the upper intellectual ranks of true nuetrals - people who have to justify to themselves why they are behaving in this way - and even that isn't that unusual of a belief system.
6) Alignment defines your personality.
This is one of the most annoying ones for me because it so misses the point. Yes, certain alignments are more likely to attract people of particular personalities than others, but by no means does having a particular personality define a characters alignment any more than a alignment defines a personality. Not only can you have a dour taciturn person of any alignment, or a shy or gregarious person of any alignment, but you can have a miserly greedy person of any alignment whatsoever. Now granted, a miserly greedy NG is more likely to part with his coin in extremis of his neighbor's need than a miserly chaotic neutral, but the same feelings and urges can drive them both. And there is every chance that a generous CN gives more money away than a miserly nuetral good (but there might be worlds of difference between the amount of concern for thier fellows and compassion they are actually showing when they do it).
A personality is far to complex to model with any combination of wisdom, intelligence, charisma scores and alignments. The combination might be very suggestive of certain things, but it doesn't impose a whole lot.
7) Good aligned people never fight with each other.
Good aligned people find just as likely to find cause to argue about and even (if they are of a martial bent, as most good aligned D&D characters are) go to war over things as any other aligment. Nothing prevents two lawful good people with a difference of opinion from fighting to the death over something if each has a different take on it and each feels duty compels them to a mutually exclusive choice.
If we are to believe in a cosmology in which law is as far from chaos as evil is from good (and I don't, but most alignment maps represent it this way), then we must believe that an Archon finds an Eladrin every bit as distasteful and worthy of destruction (or alliance) as an Devil. Why should a lawful good find chaos more pardonable than evil?
In the default cosmology, a blood war in the heavens makes as much sense as a blood war in the hells.
(This is one of several reasons I prefer to ignore the default cosmology.)
