Things in the official Call of Cthulhu publications that ruin the atmosphere and make the Mythos more ordinary at a stroke:
Calling members of the Great Race of Yith "Yithians".
"The Great Race" a) is more authentic - that's how Lovecraft referred to them, and b) retains the overblown ponderous pomposity of the background. "Yithian" sounds more 1970s than antediluvian, robbing the race of all it's mystery. I might fear a member of something everyone calls "The Great Race". A "Yithian" is as scary as a "Smurf".
Calling The Insects from Shaggai "Shans". It seems obvious to me the plural of Shan would be "Shan" - as in "Fear The Shan". I don't know precisely why, but pluralizing "Shan" makes it ordinary and unscary. Probably because is sounds less pompous.
Calling a Hound from Tindalos "A Tindalos". It should be "A Hound" or, if one absolutely must trivialize the most dangerous thing you could ever have tracking you "A Tindalosi". Again, it has to do with the pompous sound of the name.
Trivialize the name and you trivialize the monster and the threat it is supposed to pose. That is why we routinely make the first order of any war the coining of a dismissive diminutive for speaking of the enemy.
And the Threat of The Mythos should never be trivialized.
