There are many synonyms for the phrase "was in error." Some examples include "made a mistake," "was incorrect," "was mistaken," "misinterpreted," "misunderstood," "was wrong," "erred," and "blundered." These phrases can be used to express the same concept of being incorrect or mistaken about something. It is important to use these synonyms to avoid repeating the same phrase over and over again in writing or conversation. Using synonyms can also help convey different levels of severity or tone, depending on the context in which they are used.