Illusion of transparency
Words are a means of communication, but they don't in themselves contain meaning. The word apple is just five letters, two syllables. I use it to refer to a concept and its associations in my mind, under the reasonable assumption that it refers to a similar concept and group of associations in your mind; this is the only power words have, great though it may be. Unfortunately, it's easy to lose track of this fact, think as if your words have meanings inherently encoded in them, leading to a tendency to systematically overestimate the effectiveness of communication.