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.”

Thanks Alex. Very useful and interesting. It's in my favorites now.

If it's archaic you'd have to ask what's the therefor there for then?

My favorite is the expansion on the very first entry: He did a complete 365 degree turnaround.

Many years ago I heard a sporting commentator make that error. He said something like "he did a complete 360 and ended up running in the wrong direction..."

In my favorites as well. Nice and clear.

Enormity, some people insist, is improperly used to denote large size. They insist on enormousness for this meaning, and would limit enormity to the meaning “great wickedness.” Those who urge such a limitation may not recognize the subtlety with which enormity is actually used. It regularly denotes a considerable departure from the expected or normal <they awakened; they sat up; and then the enormity of their situation burst upon them. “How did the fire start?” — John Steinbeck>. When used to denote large size, either literal or figurative, it usually suggests something so large as to seem overwhelming <no intermediate zone of study. Either the enormity of the desert or the sight of a tiny flower — Paul Theroux> <the enormity of the task of teachers in slum schools — J. B. Conant> and may even be used to suggest both great size and deviation from morality <the enormity of existing stockpiles of atomic weapons — New Republic>. It can also emphasize the momentousness of what has happened <the sombre enormity of the Russian Revolution — George Steiner> or of its consequences <perceived as no one in the family could the enormity of the misfortune — E. L. Doctorow>.

http://www.merriam-webster.com/dictionary/enormity

Language is not static in its forms or meanings or usage. Either in time or in place (dialects). It is a fluid, living, ever changing creature. It creeps, downwards, upwards, and sideways.

'Enormity' has simply shifted in meaning. Interesting that another word of similar meaning, 'wicked' is undergoing a revision of meaning, in this case a reversal from 'bad' to 'good' through its use in an ironic, rebellious way by the youth generation of the 90s. The English language is wicked in its enormity don't you think? Awesome!