If Imaam makes mistake in basic actions like Ruku' and Sujood, the follower should correct him by saying out loud "Subhanallah". If the mistake is realised by a female follower, she can alert by clapping.
Sahl b. Sa'd al-Sa'idi reported:
The Messenger of Allah (may peace be upon him) went to the tribe of Bani Amr b. Auf in order to bring reconciliation amongst (its members), and It was a time of prayer. The Mu'adhdhin came to Abu Bakr and said: Would you lead the prayer in case I recite takbir (tahrima, with which the prayer begins)? He (Abu Bakr) said: Yes. He (the narrator) said: He (Abu Bakr) started (leading) the prayer. The people were engaged in observing prayer when the Messenger of Allah (may peace be upon him) happened to come there and made his way (through the people) till he stood in a row. The people began to clap (their hands), but Abu Bakr paid no heed (to it) in prayer. When the people clapped more vigorously, he (Abu Bakr) then paid heed and saw the Messenger of Allah (may peace be upon him) there. (He was about to withdraw when) the Messenger of Allah (may peace be upon him) signed to him to keep standing at his place. Abu Bakr lifted his hands and praised Allah for what the Messenger of Allah (may peace be upon him) had commanded him and then Abu Bakr withdrew himself till he stood in the midst of the row and the Messenger of Allah (may peace be upon him) stepped forward and led the prayer. When (the prayer) was over, he (the Holy Prophet) said: 0 Abu Bakr, what prevented you from standing (at that place) as I ordered you to do? Abu Bakr said: It does not become the son of Abu Quhafa to lead prayer before the Messenger of Allah (may peace be upon him). The Messenger of Allah (may peace be upon him) said (to the people) around him: What is it that I saw you clapping so vigorously? (Behold) when anything happens in prayer, say: Subhan Allah, for when you would utter it, it would attract the attention, while clapping of hands is meant for women.
While, if the mistake is made in the recitation of Qur'an, the follower should point out the mistake by reciting the correct Ayaah.
If the Imaam still doesn't realise, he should be explained after finishing the prayer. If the meaning of recitation made by the Imaam had contradicted the meaning of actual Ayah, the prayer has to be repeated.
If a female follower detects the mistake made by the Imaam, she can alert the Imaam by clapping. She should not read out as it may go well against the above hadith.
Further reading: Correcting the mistakes of the imam’s recitation in a surah other than al-Faatihah.
ۚ وَاللَّهُ عَلِيمٌ حَكِيمٌ (And Allah is the All-Knower, the Wise)
اللَّهُمَّ انْفَعْنِي بِمَا عَلَّمْـتَنِي وَ عَلِّمْنِي مَا يَنْفَعُنِي