What if you leave a wajib act in prayer?

Question: What if someone had the intention to make Sajdatul sahu, made salam, and forgot and did something which invalidates the prayer. Would they have to make up the prayer if the time did not pass? or is this a different case?

Because I heard if you leave out a wajib and you do not repeat, it is Makruh Tahrimi, but if you leave out Sajadatulsahu by forgetfulness then it is not makruh tahrimi if you do not repeat the prayer. 

Al-Salam `alaykum

This question was
Answered by Sidi Faraz previously (See full email below.) The answer he wrote was:

If a necessary act is forgetfully omitted but one failed to perform the forgetfulness prostrations, one must repeat the prayer within the time. [ Gifts]

Abu Usamah