This is more of a list than a rant, since I have less time than usual this morning.
And for the record, though it's less frequent, I have seen these words misused in published fantasy.
taught, taut, taunt
The word that most fantasy authors mean is "taut," or "stretched tight." "His muscles were taut." However, I keep seeing "His muscles were taught" or worse, "taunt." A taunt is an insult or jibe, especially one that's meant to goad someone else into responding. It has nothing to do with tightness. And neither does taught; it just sounds the same.
I keep seeing: "He reigned in his horse." No. Rein is the word for the part of the bridle the fantasy author usually means. Reign is used in the sense of someone ruling, like a king's reign. Unless you literally mean that the rider rules over his horse, you want "He reined in his horse." This is so common that I thought at first it might be a British usage, but I have been unable to find evidence of that anywhere, so I think it's simply a really common mistake.
These are two distinct words that don't sound the same. In most cases, I think it's a slipped finger in typing and a too-swift editing that's to blame. Shin as a verb means to climb by "gripping and pulling alternately with the hands and legs," to hit someone in the shins, or to move quickly on foot. Shining is the participle form of shine. Look carefully, either when you type the word or before you hit the spell-check key to accept the word.
This is "leg armor below the knee," not armor for the arms.
Dictionary.com gives this one as "a long tunic made of chain mail." It can't be used for any old tunic, or for a surcoat; that's the tunic a knight wears over his armor, one that might bear his coat of arms to enable the audience to tell who's who in a joust.
These words are opposites, but in practice, many writers use "flaunt" for both senses. Flaunt means to show off, to blare, to blaze with color. A princess is flaunting her wealth if she wears jewels on her neck and rings on every finger. Flout means to suppress or ignore. Someone is flouting the laws if he breaks them. Many fantasy authors write "flaunt the laws," which literally means nothing and figuratively gives quite a different image than they want.
This is a name that people like to throw in when they're feeling really impressive, thinking it's just any old jewel name. In reality, it's a milky variety of quartz. Having armor or a sword made of chalcedony would probably be heavy.
The blood of angels. It means apparently the blood of just about everyone else, though. I would advise against using it that way. To quote Ursula K. LeGuin, it's "the infallible touchstone of the seventh-rate."
All it really means is supernatural or eerie. There are plenty of words that do the same thing and sound less pretentious about it.
thee, thou, thy and thine
Most fantasy authors don't use the second person singular in English correctly. First of all, it's the second person familiar; "you" was actually the formal second person, the kind you would use to address a stranger. Using "thee" and "thou" to someone like a king (who isn't a friend) is inappropriate. Second, "thou" is the subject and "thee" the object.
Right: "Thou art the image of thy father." "I am with thee."
Wrong: "Thee art my enemy." "I will hunt thou down."
"Thy" is the formation that corresponds to "my," and in strictly correct terms is only used before words that do not begin with vowels or the letter h. For that you need "thine." (It corresponds to the ancient usage by which someone would say "my castle" but "mine eyes" or "mine heart.")
Right: "thy book," "thine heart," "thine enemy."
Wrong: "thine skin," "thy eyes," "thy hair."
Many people also don't know the correct verb forms to use with the second person singular, so stick -est on the end of any verb and think it works. It doesn't. This is a short list of the special verb forms.
present tense: art
past tense: wert
present tense: hast
past tense: hadst
Most verbs follow "know":
present tense: knowest
past tense: knew
And so on.
This is third person singular present tense (though it gets used for past tense, as well as second person everything).
Right: He hath made the world.
She hath done it.
Wrong: Thou hath loved me.
He hath an old house before it was destroyed.
Reddish-brown in color. Bay horses have black manes and tails. Something is not "bay" if it's black. Nor is a bay horse with a white mane and tail normal, or one with a silver mane and tail.
This is a bright red or orange color. For some reason, many people want to use it as a shade of green. (I suspect it might be the "ver-" prefix, which a lot of people think is the same as the one in verdant).
This is a rarer mistake, but one I thought I would mention. Wrought is one of those irregular past participles that got left behind when its verb changed, like molten, which was once identical in meaning to melted. Wrought's verb form is work. "He wroughts" and "he wroughted" do not exist.
This means reddish brown to brown, not bright red. Some fantasy authors use it interchangeably with "copper" or "scarlet" hair (themselves quite different shades). Then there are auburn eyes, which would look very strange. Decide what shade your character's hair is, and then stick to using words that could reasonably be used to describe it.
This is the only adjective in English which changes with gender, I believe. Blond is for males, blonde for females. Be very careful, especially when using this to introduce a character, or you can well have half your audience thinking in the wrong direction.
Short list, but, I hope, useful.