One remembers that during a raka’a one did not recite al-Fatihah

If the Imam remembers in the final tashahhud (sitting of the prayer)
that he had recited at-tahiyyaat (the greetings mentioned during the
sitting) in the beginning of the rak‘ah instead of Al-Fatihah, he
should stand up and offer another rak‘ah, which is correct in
exchange for the one he performed incorrectly and did not recite
Al-Fatihah. This is because the Prophet
(peace be upon him)
said:
There is no prayer for the one who did not recite (in it)
Al-Fatihah (the opening chapter of the Qur’an).

  (Sahih Al-Bukhari, No. 723)

It is also required for the followers to follow him even though it
would be the fifth rak‘ah for them. If they do not understand and do
not stand up, and say “subhan Allah” to indicate to Imam that he is
error, the Imam should inform them by hand movements to the right
and left to tell them that he did it purposefully and to indicate to them
to stand up.

However, if something like this happens to one of the followers
behind the Imam, his prayer will be correct as long as he followed his
Imam.

The evidence for this is the Hadith of Abu Bakrah when he entered
into the prayer in the position of ruku‘ and did not recite Al-Fatihah.
The Prophet
(peace be upon him) said to him:

May Allah increase your endeavor. You do not need to repeat.
(Sahih Al-Bukhari, No. 750)

If the follower forgets to recite Al-Fatihah or is ignorant of its
obligatory nature or finds the Imam in the state of ruku‘, then his
rak‘ah will be considered as complete and his prayer correct. He
does not need to repeat the rak‘ah, as he is his excused due to
ignorance or forgetfulness or if he missed catching the qiyam (the
position standing upright). This is the opinion of the majority of
scholars.¹

Fatawa Islamiyyah by Ibn Baz, 1/263.