The eminent Muslim scholar, Sheikh Sayyed Sabiq, states in his well-known book, Fiqh As-Sunnah: “It is allowed for men to say ‘Subhanallah’ (Glory be to Allah) and for women to clap if there is some need to do so (such as alerting the Imam to a mistake or informing someone that he or she may enter the room or to warn a blind person, and so on). Sahl ibn Sa`d As-Sa`di (may Allah be pleased with him) related that the Prophet (peace and blessings be upon him) said: “If someone is faced with something during the Salah (Prayer), he should say ‘Subhanallah.’ Clapping is for women and saying Subhanallah is for men.” (Reported by Ahmad, Abu Dawud, and An-Nasa’i)
If the Imam forgets a verse, it is permissible for a person praying behind him to remind him of it, regardless of whether the recitation is a part of the obligatory recitation or not. Ibn `Umar (may Allah be pleased with them both) reported that the Me
ssenger of Allah (peace and blessings be upon him) prayed and had some confusion in his recitation. When he finished, he said to `Umar: “Were you present with us [during the Prayer]?” He replied: “Yes.” So, the Prophet (peace and blessings be upon him) asked him: “Why then you didn’t correct me?” (Reported by Abu Dawud)”
Shedding more light on this issue, the following is the fatwa issued by Sheikh M. S. Al-Munajjid, a prominent Saudi Islamic lecturer and author:
“There are two stages of correcting the Imam:
1. Cases of obligatory correction: Such correction takes place concerning something which will invalidate the Prayer if done deliberately. If Imam makes a grammatical mistake which changes the meaning of Surat Al-Fatihah, then he must be corrected, because this type of mistake which changes the meaning of Al-Fatihah invalidates the Prayer. Similarly, if he omits a verse of Al-Fatihah, he must be corrected, because that invalidates the Prayer.
2. Cases of Mustahabb (recommended) correction: This is when the Imam makes a mistake which does not invalidate the Prayer altogether, but it does undermine the perfection of the Prayer. So, if the Imam forgets to recite another Surah after Al-Fatihah, alerting him to this fact is Sunnah.
The evidence for that is the words of the Prophet (peace and blessings be upon him): “I am a human being like you, I forget as you forget. If I forget, then remind me.” (Reported by Al-Bukhari)
On one occasion when the Prophet (peace and blessings be upon him) became hesitant in his recitation, he said to Ubayy ibn Ka`b (may Allah be pleased with him) “What stopped you?”, i.e., what stopped you from correcting me? This indicates that correcting the Imam is something that is essential. See Ash-Sharh Al-Mumti’ by ibn `Uthaymeen, part 3, pp. 346-347.”