“But that would be putting the clock back,” gasped the governor. “Have you no idea of progress, of development?”

“I have sent them both in an egg,” said Caspian. “We call it ‘Going Bad’ in Narnia. This trade must stop.” 

The King said it better than I ever could.  He isn’t just talking about the heartbreaking slavery trade, but about the whole concept of worldly ambitions and what comes from it.  This is heavy stuff in Narnia!  

Using the word "girl" around talking dogs. 

I'm just going to leave it at that....  

Watching a squirrel go to collect a nut from his store! (From Prince Caspian — I'm away from home and don't have the books with me, or I'd put in the exact quote.)

Also, being persuaded to dress up in a lion's skin and pretend to be Aslan isn't a good idea...

Being disrespectful of a mouse's tail (especially Reep's).

Comparing riding a Talking Horse to riding a donkey!!

How about calling on a god you don't believe in? It's basically like praying but it's not sincere.

Eustace sneaking the water rations at night on the Dawn Treader and taking more than his share was one of the worst things he did.  And of course Reepicheep caught him doing it.  It was one of the most interesting parts of the book.

Putting a saddle on a Centaur or a unicorn! (If you value your life, of course).

From The Lion, the Witch, and the Wardrobe / Peter’s First Battle chapter:

Peter, still out of breath, turned and saw Aslan close at hand. 
“You have forgotten to clean your sword,” said Aslan. 
It was true. Peter blushed when he looked at the bright blade and saw it all smeared with the Wolf’s hair and blood…

As memory serves, this is the only rebuke directed at Peter during his service in Narnia. For a knight and a High King, however, unreadiness for battle is a major no-no. 

@reshpeckabiggle That's actually less a Narnian specific thing and more of a real world thing. If the sword is all sticky, it'll get stuck in the sheathe.  

Perhaps another is cutting down trees where the dryads live. If a tree is cut down, a dryad dies.

@jasmine_tarkheena 

… which begs the question: are there trees that have no Dryad?  Is there a way for a discerning Narnian to know?

@reshpeckabiggle 

That's a good question. I know that the male, also known as "wood gods" live in oak, elm, holly, and rowan trees and the female live in beech, birch, willow, and larch trees. The rest like maple and myrtle and any fruit tree any nut tree we can only speculate if the dryad would be male or female.

Posted by: @reshpeckabiggle

are there trees that have no Dryad?

Since the Narnians have structures made of wood, like the Dawn Treader and the Beavers' dam, yes, though I suppose the wood could be imported. Not sure how that could happen during the White Witch's reign however. It's implied that everyone from outside was afraid to enter Narnia then.