From the first part of the entry on “incredible” in my book:
The other day I heard a film reviewer praise a director because he created “incredible characters,” which would literally mean unbelievable characters. What the reviewer meant to say, of course, was precisely the opposite: characters so lifelike as to seem like real people. Intensifiers and superlatives tend to get worn down quickly through overuse and become almost meaningless, but it is wise to be aware of their root meanings so that you don’t unintentionally utter absurdities.
The problem is that when people use these words in this way they are always exaggerating. They mean something like “almost incredible.” This leads to all sorts of contradictory statements.
A woman writing about her cheating boyfriend to advice columnist Amy Dickinson says today, “He often denied evidence I found of his possible dalliances and made me believe it was all an unbelievable coincidence.”
The entry for “incredible” in the book inspired National Public Radio’s Scott Simon to write the following blurb, which you’ll find on the back cover of the second edition of Common Errors in English Usage:
I’d call Paul Brians’ book incredible, fabulous, or fantastic, except thanks to him, I know now that none of those words are what I really mean. Let’s just say that Common Errors in English Usage is the most cheerfully useful book I've read since the Kama Sutra.