Mistakes will always be made in French, and now you can learn from them.
Wrong? On est allés
Right? On est allé
Explanation: The French pronoun on is a little weird. It literally translates to "one" as in "one shouldn't do that," but can also mean "we," "you," "they," or "people in general." One often hesitates over agreement when on replaces one of these plurals - since on has to take the third person singular form of the verb, it might seem like agreement should also be singular - in other words, that there is no agreement. In fact, agreement with whatever subject on replaces is optional - either way is correct.
Nous sommes allés au cinéma. (We went to the movies.)
On est allé au cinéma. / On est allés au cinéma.
Sont-elles prêtes ? (Are they ready?)
Est-on prêt ? / Est-on prêtes ?
On - indefinite subject pronoun