Ambiguity forces readers to determine the meaning based on insufficient evidence. Important Clarity is more than an absence of ambiguity. Writing clearly is a little like testifying under oath. It's a tall order. Ambiguity has a way of creeping into the most unlikely places.
Clear writing is a little like testifying under oath.
Though it's probably clear from the context that I intended the second meaning, readers should never be forced to rely on context. I thought it was perfectly clear. So did the folks who signed off on it, as well as another writer and a proofreader. But what we discovered when we tested it was that it wasn't clear at all. Head-slappers often result from Humpty Dumpty writing.