The phrase "under an error" can refer to someone who has made a mistake or is mistaken about something. There are many synonyms that can be used to describe this state of being, including "misguided," "misinformed," "mistaken," "wrong," "fallacious," "erroneous," "inaccurate," "incorrect," "untrue," "false," or "in error." Each of these words conveys a slightly different nuance of the idea of being under a mistake, whether it is intentional or unintentional. By choosing the most appropriate synonym, you can accurately convey the meaning of being under an error in your writing or speech.