It's like gang-stalking--its not that there's something being introduced, but rather that the subject sees relations that are not objective relations (like, for instance, the relation between temperature, pressure, and state change). Typically, however (and I can't imagine a case where this didn't happen), the relations are social in character--and since social relations are subjective to the extent that all the social world is not expressly a fact, it can be difficult to differentiate between an illusion and a reality: people imagine their partners are cheating on them, whether or not its true. And there are many things we do not know about the social world around us; but, statistically speaking, nobody has ever actually been gang-stalked.