This post is taken from a book titled A Dictionary of Contemporary American Usage (Random House, 8th printing, 1957).  Given how old it is, we may wonder how "contemporary" it remains, but some of the entries are still valid and interesting.
Flammable vs. Inflammable   Notwithstanding that some people have distinguished these two terms by saying that the former means combustible and the latter explosive, they are in fact synonomous. On trucks with this warning, "it's just a matter of lack of space or paint, a desire to use bigger letters, a stylistic preference, or [I like this one] a recommendation of the underwriters who found that many people thought that inflammable meant not flammable."
Egoism vs. Egotism    "Egoism is the name for the philosophical belief that self-interest is the true end of moral actions. As such, it is opposed to altruism. The egoist relates all questions to himself...  Egotism, the too-frequent use of I in conversation, hence boastfulness, too much interest in one's own doings to an exclusion of interest in others, self-conceit, selfishness, is the more common word... Egoism and egotism do not, necessarily, have to have anything to do with each other. A self-effacing man may be an egoist.  An egotist may have no interest in any philosophy whatsoever."
Discreet vs. Discrete    "Discreet means wise or judicious in avoiding mistakes, prudent, circumspect, cautious, not rash.  Discrete means separate, detached from others, distinct by itself."
Connotation vs. Denotation    "To denote is to mark, indicate, designate.  To connote is to signify in addition to the primary meaning, to involve as a condition of accompaniment. A word denotes the indispensable minimum of definition. A word connotes all of the attributes which are not denoted but which are associated with it. Thus the word pig denotes a young swine of either sex.  It connotes filth and gluttony and high-pitched squealing" and etc. 
Petty vs. Paltry vs. Trivial    "All apply to something so small as to be unworthy of serious attention. Petty connotes a good deal of contempt. Paltry is even stronger. It derives from a word meaning a rag and suggests something so mean and worthless as to be dispicable. Trivial means slight and insignificant. It is used especially to mark the incongruity of the trifling when compared with the serious and important. It is not as scornful a word as the other two."
...AND THAT'S AS SERIOUS AS I INTEND TO BE TODAY.