Is the follower obligated to follow the Imam in an unintentional increase in the number of Rak'ahs, or should he sit and wait for the Imam to say the Tasleem?

If the Imam makes a mistake and adds an extra rak'ah, and the ma'mum (follower) becomes aware of this addition, then it is not permissible for him to follow the Imam in the extra rak'ah; otherwise, his prayer will be invalidated. The ma'mum has the choice between leaving the Imam and concluding his prayer, or waiting for the Imam to conclude his prayer and then concluding with him. This latter option is the more correct one, due to the saying of the Prophet, peace and blessings be upon him: "O people! I am your Imam, so do not precede me in bowing, nor in prostrating, nor in standing, nor in departing."