The two biggest misconceptions that come to mind immediately...
Susan goes to hell at the end of The Last Battle (no she doesn't, she's still alive on Earth).
The White Witch has black hair (it's never given except in Pauline Baynes's drawings).

Two more misconceptions, one of which is mentioned in Prince Caspian.

1. That the White Witch could be revived despite being killed by Aslan at the first Battle of Beruna. Only in their nightmares, if that.

2. That old chestnut, mentioned elsewhere on this board, that LOTGK and the White Witch are the same person, which owes much to the BBC's economies in typecasting Barbara Kellerman in Narnian villainess roles.

I think another example may be that Lucy sees Susan in the spellbook in VDT - perhaps due to the BBC (and later the Walden version) changing the scene from the book in the book she just sees an incredibly beautiful woman)

Actually Lucy does see Susan in the VDT spellbook. Check out chapter 10, the Magician's Book. She sees herself saying the spell, then the consequences in the Narnian lands, then Susan "who had always been the beauty of the family" coming home from America, with a nasty look on her face because Susan has become plainer than Lucy, herself.

Both BBC and Walden seized on this particular part of Lucy examining the book, because it was far easier than depicting the possible consequences to Narnia, such as wars and lands being laid waste. Not to mention the spell Lucy actually did say, directly afterwards, the one involving eavesdropping on Marjorie Featherstone.

I keep stumbling upon angry 'fan' posts on facebook and other sites about how The Magician's Nephew should be the next movie made because it's the next book in order and if they do SC next they'll be going out of order. I'm calling this a straight out misconception, as opposed to attributing it to chronological vs publication reading orders because such comments almost always specify that it is the next book in the series where as even with Chronological order MN is long before VDT

That's interesting. I've keep seeing comments from fans on various movie news sites saying that MN should be the next film made. But I haven't seen any who think it's the next book in the series. I'm not sure how anyone would think that, maybe the two reading orders have them so confused that they made up a new one.

The only misconceptions I can think of are Susan being left out of Aslan's country (which as already been mentioned) and that the Narnia books are a allegory(I know some fans may disagree about whether this is a misconception or not)

With the Susan misconception I think it stems from a combination of people not being familiar with or not understanding the book and that modern authors like J.K. Rowling and Philip Pullmen have openly criticized it.

Then with the idea that Narnia is a allegory. I think sense Lewis was and still is one of the most well known christian apologists and there are definitely some christian themes in the chronicles, some people simply assume that the Narnia book are an allegory of some kind. Of couse this isn't always the case, but I have known people who thought Narnia was an allegory for these exact reasons.

That's interesting. I've keep seeing comments from fans on various movie news sites saying that MN should be the next film made. But I haven't seen any who think it's the next book in the series. I'm not sure how anyone would think that, maybe the two reading orders have them so confused that they made up a new one.

After VDT was made there was some dispute between the C.S.Lewis Estate and Walden about which movie would be made next. Walden's Michael Flaherty seemed to prefer MN, there was talk of the money needed to film MN being available, just as someone died, and the IMDb board for that movie was duly made up to accompany the already existing SC one, last changed on 9 March. That dispute resulted in moratoriums left, right and centre, the options on the movies having expired.

The main argument for doing MN next was that it was a prequel, that it was a more popular book than SC, and that there was more likelihood of its earning plenty of money. However, if Walden had got all its own way, then Will Poulter, who featured as Eustace in VDT, would most certainly have not returned to play Eustace in SC, given the time lapse between movies Walden had experienced in producing PC and VDT.

I've a strong suspicion that Walden also wanted to make more use of Tilda Swinton as Jadis for MN, and to keep the movies together that featured her though nobody ever said so. One of the reasons why VDT annoyed fans was because of the inclusion of the White Witch as a nightmare of Edmund's, or was it Caspian's nightmare? It was one cameo too far, anyway.

Narnia fan 7, the word "allegory" has a specific and narrow meaning in English Literature. The best known example in English Lit is "The Pilgrim's Progress".
Lewis would have known the meaning of the word, as he was a University Lecturer in English Literature!
Usually when someone refers to Narnia stories as allegory, what they are thinking of is really representation, symbol, or allusion.

Wagga, the person who died (I know who you mean) was no longer working on the Walden films any more, but spoke publicly about making MN. This misled a lot of people.

"Wait. There are more than three books?!"

"There's no green mist in the book?"

"The White Witch isn't in Prince Caspian?" etc.

Most of the things I come across are ideas people have got from watching the films and not having read the books. I often encourage people to read them, but most people don't want to. The attitude seems to be that they're children's books, how good can they be? Another misconception! Of course, Lewis has an answer to that in his lovely dedication: "But some day you will be old enough to start reading fairy tales again."

I remember when the first promotional material came out for LWW, before I'd discovered NarniaWeb, that I found a bookmark saying "There are a thousand stories in the land of Narnia. The first is about to be told ... " It confused me a lot, because though I'd read the Chronicles in something close to publication order it was completely by accident. Until NarniaWeb educated me I genuinely thought MN came first.

Here's one Lewis himself observed:

Some people seem to think that I began by asking myself how I could say something about Christianity to children; then fixed on the fairy tale as an instrument; then collected information about child-psychology and decided what age-group I’d write for; then drew up a list of basic Christian truths and hammered out ‘allegories’ to embody them.

This is all pure moonshine. I couldn’t write in that way at all. Everything began with images; a faun carrying an umbrella, a queen on a sledge, a magnificent lion. At first there wasn’t even anything Christian about them; that element pushed itself in of its own accord.

- Sometimes Fairy Stories May Say Best What's to Be Said (1956)

Here's one Lewis himself observed:

Some people seem to think that I began by asking myself how I could say something about Christianity to children; then fixed on the fairy tale as an instrument; then collected information about child-psychology and decided what age-group I’d write for; then drew up a list of basic Christian truths and hammered out ‘allegories’ to embody them.

This is all pure moonshine. I couldn’t write in that way at all. Everything began with images; a faun carrying an umbrella, a queen on a sledge, a magnificent lion. At first there wasn’t even anything Christian about them; that element pushed itself in of its own accord.

- Sometimes Fairy Stories May Say Best What's to Be Said (1956)

Great quote which I hadn't heard before. I wasn't aware that Lewis himself addressed this. Really good to know. I believe a lot of people could learn something from this. There is one camp of people who should stop assuming he wrote the Chronicles to indoctrinate children. There is another camp of people who want to use the Chronicles to evangelise. They are stories imbued with the author's faith and should be taken as such.