Words are not static: meanings shift; new ones arise; old ones become archaic - some words even mean their own opposites; historical origins are forgotten; and we add new ones, usually cognates of existing words. To echo a cliche modern complaint, whoever thought "friend" would become a verb? And who uses "nonplussed" correctly nowadays?
This poses challenges for the secondary world fantasy writer. In all probability, a totally accurate story - one using no words that owe their existence to specific incidents in Earth history - would be almost impossible to write for its limited vocabulary. Just removing the words invented by Shakespeare (tranquil, rant ...) would probably be a challenge.
So we're generally left with avoiding words that have an obvious real world connotation. For instance, "spartan" has a fairly direct and obvious connection with the Greek city-state and the ... well ... spartan lifestyle of its inhabitants. On the other hand, would we look twice at "gypped," which is derived from gypsy?
Jane Lindskold has a neat example of taking this in the other direction. In her Firekeeper books, one of the historical rulers is a queen named Zorana, known (among other things) for her plain, straightforward manner. So, when things are unnecessarily convoluted or ornate, the characters now and again refer to them as being "unzoranic." This is the only word coined in the books, and it isn't overused: instead, it is a perfect little tidbit of worldbuilding.
Then again, what about words that are period / historical, but that feel too modern for the setting? In looking up a complete list of Shakespeare's inventions, I see "advertising" and "skim milk" - neither of which feel like Elizabethan words to me! It's up to the writer, the word and the moment to decide where they can fool the reader ... into believing the truth.