I was taught this on a Technical Writing course and it changed the way I thought about my audience, whether I was speaking or writing.
If you are trying to communicate an idea to someone and they misunderstand you, you cannot blame them. You have either not explained it as clearly as you thought, or you have misjudged your audience.
Sometimes people misread something you’ve written, or miss out a crucial part altogether. Often there is still blame on your part because you could have written more clearly or made your document easier to navigate.
Update: XKCD have kindly made a cartoon to illustrate this very topic.