The phrase "in error" means to make a mistake or error. Some synonyms for this phrase include mistakenly, erroneously, incorrectly, inaccurately, and faultily. When someone is "in error," they may be misguided, mistaken, or incorrect in their viewpoint or actions. Other related synonyms might include misjudged, miscalculated, misunderstood, or misinterpreted. When using these synonyms, it is important to be clear about the nature of the mistake or error being referenced, as the context of the situation may impact the appropriate vocabulary choice. Ultimately, choosing the right synonym for "in error" can help to convey a more precise meaning and intent in written or verbal communication.