I am a bit confused about an issue. As far as I know, Surah Fatiha is obligatory to recite for the Shafi'is whereas the sura after it is a sunna (and not a main sunna for which a forgetfulness prostration is required).
However, in Sheikh Nuh's translation of the Maqasid, he adds in 3.13(4) ''ommitting the sura necessitates a forgetfulness prostration''.
So what IS the position?
This is important as there might arise emergency situations where you need to make the prayer as brief as possible.