ash-Shaykh, al-'Allaamah, Rabee' bin Haadee al-Madkhalee (حفظه الله) was asked:

Question: Does one pray behind an imaam of a masjid that makes qunoot in Fajr and makes du'aa and raises his hands and do we follow him (in that)? And if they pray salaatul-Istisqaa' in the masjid, do we pray with them?

Answer: If the imaam prays and makes qunoot, then make qunoot with him. The follower differing and opposing the imaam, even if he sees the salaah of the Imaam is not correct according to his madhhab, it is correct in the madhhab of this Imaam, but it is not correct with you – pray behind him. The Messenger (Sallallaahu 'alayhi wasallam) ordered with the salaah behind him and said: "If they lead you in the salaah correctly, then they and you will receive the rewards but if they make a mistake (in the salaah), then you will receive the reward and the sin will be theirs." (1) So you pray behind him.
The Salaf used to, meaning Ar-Rasheed (the ameer) came for the Hajj and he stopped in Madeenah and had hijaamah (cupping) done, and he asked (Imaam) Maalik: 'I was treated by hijaamah, do I pray without (a new) wudhoo'.' He said: 'Yes, pray!' So he prayed with the people and did not make wudhoo' due to hijaamah.
With the Ahnaaf (the Hanafees), hijaamah nullifies wudhoo'. So it was said to Aboo Yoosuf, 'How did you pray behind Ar-Rasheed and he had hijaamah done and did not make wudhoo'?' He said: 'Subhaanallaah! (He is) Ameerul-Mu'mineen!'
And there is for Shaykhul-Islaam Ibn Taymiyyah some speech, that you pray behind the Imaam. If you differ and him in an issue; you see that what he has is false, his salaah is not correct, but he has usool (fundamental principles) and he has adillah (proofs) – he sees his salaah is correct – then pray behind him even if you don't see the correctness of his salaah, then pray behind him – except if you are sure that he did not make wudhoo'. (like) he said to you: 'I do not make wudhoo' and I pray without wudhoo'.' Then his salaah is null and void, for you and him."
(1) Collected by al-Bukhaaree #662, from the hadeeth of Aboo Hurayrah
[Eng. 1/663]