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

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)