3. If the Imam mistakenly adds a third Sajdah, or stands for an extra rak'ah after having completed the final sitting, the follower does not follow the extra act; he waits and may remind the Imam.
4. If the Imam returns before attaching the extra rak'ah to a Sajdah, the follower gives Salaam with the Imam.
5. If the Imam completes a Sajdah in that extra rak'ah after the valid final sitting, the follower gives Salaam alone rather than following the Imam into the extra prayer-act.
The final sitting matters here: these rules concern an extra addition after the Fard prayer's final sitting has already been completed.