When a word is displayed with a red underline, it means that this word is wrongly spelled or not in the dictionary. In the latter case, the word can be saved into the dictionary in Check Spelling.
But I have found that some words (either correctly spelled or personal names) are red underlined, however, it can not be saved into the dictionary because it never shows the word when doing Check Spelling. What is the problem?
Edit: Here is a short example:
In \cite[p214]{Kosko}, Kosko presents an inexact oval as a fuzzy ellipse to dispute a Lindley's (can not be checked) claim \cite{Lindley}: \textit{Probability is the only sensible description of uncertainty and is adequate for all problems involving uncertainty. All other methods are inadequate}.
Obviously, Lindley's claim is wrong because not all uncertainty issues can be handled by probability. However, the fuzzy ellipse by Kosko is inadequate because it lacks mathematical rigor to define the exact degree of elliptical roundness.