Forgetfulness
In Doubt
Not Knowing
Repeating the Prayer in Full
Sahu Sajda ( Prostrations of Correction )
While in prayer :
Bukhari book 8, #394
" ..... if I forget remind me and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly and finish it and do two prostrations (of Sahu)."
After completing the prayer:
Sahih Bukhari Volume 001, Book 008, Hadith Number 398:
Narated By 'Abdullah : "Once the Prophet offered five Rakat in Zuhr prayer. He was asked, "Is there an increase in the prayer?" The Prophet said, "And what is it?" They said, "You have prayed five Rakat.' So he bent his legs and performed two prostrations (of Sahu).