The Sajda Sahuw is only requried when an Imam:
Forgets a mandatory [Faraz]. For example, forgets 1 Sajda, or forgets number of Rakat, forgets Raku, forgets Qeyam (standing), or skips by mistake next Faraz.
Read Tash-had till or beyond Darood in 2nd Rakat (while sitting) while being in 4 Rakat Faraz, where Imam is supposed to get up for 3rd Raqat.
Imam is was supposed to sit for Tash-had but he stands up.
This is mentioned in Quran to offer Sajda-Sahaw when any Faraz gets missed.
Repeating any Ayah or forgetting any Ayah or any Luqma in Salat/Prayer/Namaz does not required any Sajda-Sahaw. For instance, if you see Namaz-e-Taraveeh in Ramadan. In Namaz-e-Taraveeh Imam recites and if forgets something there is a special dedicated person behind Imam to feed or give Luqma of correction and no Sajda-Sahaw is conducted at the end.
In Jamaat/collective prayer behind Imam this does not apply to followers. This only apply to single person when he/she is offering Prayer/Salat/Namaz as single.
Also, In some cases if you gone late and you missed 1 Rakat, you required to stand (to complete your missed 1 Rakat) after Imam says
Asalam-o-alaikum Wa Rehmatullah. In that case now you are offerign Prayer/Salat/Namaz as a single; so now if you do any mistake as mentioned above then you have to do Sajda-Sahaw.
Moreover, if you by mistake followed Imam
(on Aslam-o-Alaikum Wa Rehmatullah) and moved your head by mistake, and suddenly you noticed that you missed 1 Rakat, and you need to complete that too, then you required to offer 1 Sajda-Sahaw at end of your prayer.