What is the ruling on a person praying alone behind an Imam who makes mistakes in the takbir, with the intention of immediately praying alone, and not differing from the Imam in movements?

If one prays behind an imam whose prayer they believe to be invalid, but they outwardly conform to the form of the prayer and intend to pray individually, their prayer is valid and sufficient. This is what Al-Mardawi stated in Al-Insaf concerning the ruling on praying behind a fasiq (open sinner), where he said: "And if he intends to pray individually and conforms to his actions, he does not have to repeat it. This is according to the correct view of the madhhab (definition)." This comes with a warning against obsessive doubts and believing something is invalid when it is not, as the default is the validity of imams' prayers.