There are several synonyms for the phrase "is mistaken". The most common are "is wrong", "is incorrect", and "is in error". These phrases all convey the idea that someone has made a mistake in their thinking or understanding. Other possible synonyms include "misunderstood", "misjudged", and "made an error". These words can be used in a variety of situations, from correcting a friend's misunderstanding to acknowledging one's own mistake. It's important to remember that making mistakes is a natural part of learning and growth. By using these synonyms, we can communicate our corrections and feedback in a respectful and constructive way.