What are the implications for someone who, out of forgetfulness, prayed 'Isha with the imam with the intention of Maghrib, then remembered in the middle of the prayer?

Intention (niyyah) is a condition for the validity of prayer. Therefore, if someone enters the Isha prayer with the intention of performing the Maghrib prayer, their opening takbir (tahrim) is invalid. They must interrupt the prayer upon remembering and recommence it with the intention of the Isha prayer. If they do not do so, or do not remember until after the prayer has finished, they must repeat it. If the time for the prayer has elapsed, they are obligated to make it up (qada'). However, if someone joins the Imam with the intention of performing the Maghrib prayer, having already prayed Maghrib at its prescribed time, there is no obligation upon them, according to the preponderant opinion.