While filling in a form you respond to a question that tricks you into giving an answer you didn't intend. When glanced upon quickly the question appears to ask one thing, but when read carefully it asks another thing entirely.
This is very common when registering with a service. Typically a series of checkboxes is shown, and the meaning checkboxes is alternated so that ticking the first one means "opt out" and the second means "opt in". Confusing language is often also used. Currys PC World provides a good example of this on their checkout collection page, as shown below.