Imam errs during prayer and does not know what is 
the mistake
Question

                If the Imam makes a mistake in the prayer and the 
followers try to remind him but he does not know when 
he made the mistake, what should they do?
Praise be to Allah.If the Imam makes a mistake in the prayer and misses an obligatory 
act, while the followers remind him of the mistake (by saying 
“subhan Allah”) but he does not understand what they mean or does 
not know when or where he made the mistake, and continues to 
move on to other obligatory acts which do not include the missed 
act, then there a number of opinions on this. The best of these 
opinions is that they remind him of the act by the particular 
supplication that act, e.g., saying “subhana Rabbi al-‘Azeem” if it 
was the ruku‘ or “subhana Rabbi al-A‘la” if it was the prostration, 
or Rabbighfir li if it was the sitting between the two prostrations, etc. 
(Al-Mughni 1/707)