I don't know if this has been posted before, but here's an interesting and fairly comprehensive list of errors including word pairs with subtle differences in meaning.

http://www.wsu.edu/~brians/errors/errors.html#errors

The explanations are brief and to the point, such as the following entry for enormity/enormousness: Originally these two words were synonymous, but “enormity” got whittled down to meaning something monstrous or outrageous. Don’t wonder at the “enormity” of the Palace of Versailles unless you wish to express horror at this embodiment of Louis XIV’s ego.

I found the entry for therefor/therefore [I can never keep them straight] to be particularly helpful: The form without a final “E” is an archaic bit of legal terminology meaning “for.” The word most people want is “therefore.”