The deviant sects Such as Khawarij and Rafidah, often try to undermine the Caliphate of the rightly guided Caliphs, such as Abu Bakr, Umar, Uthman and Ali, (May Allah be pleased with all of them) for their ijtihad in some matters during their rule, which appeared to be contrary to the Sunnah of Prophet(saws).
For example:
A Khawariji could accuse and disregard the Caliphate of Ali, claiming that he went against the Sunnah, when he punished people with fire. Or that he changed Islamic Capital from Madinah to Kufah, etc.
OR
A Rafidi could accuse and disregard the Caliphate of Abu Bakr, Umar and Uthman, claiming that Umar & Uthman prohibited people from hajj tamattu, or that Umar issued the command of counting triple divorce as three, etc.
But one hadeeth of Prophet(saws) is sufficient to prove the incorrectness of criticism against the ijihaad of Rightly Guided Caliphs, where we find Prophet(saws) saying follow my Sunnah and the Sunnah of rightly guided Caliphs.
Narrated Al-'Irbad bin Sariyah: "One day after the morning Salat, the Messenger of Allah (SAWS) exhorted us to the extent that the eyes wept and the hearts shuddered with fear. A man said: 'Indeed this is a farewell exhortation. [So what] do you order us O Messenger of Allah?' He said: 'I order you to have Taqwa of Allah, and to listen and obey, even in the case of an Ethiopian slave. Indeed, whomever among you lives, he will see much difference. Beware of the newly invented matters, for indeed they are astray. Whoever among you sees that, then he must stick to my Sunnah and the Sunnah of the rightly guided Khulafa', cling to it with the molars.'"
Since we dont have any khawariji on the forum, I would like to quote the report about the ijtihad of Ali, which invalidates the criticism of Rafidah against Abu Bakr, Umar and Uthman for their ijtihad.
We read: Narrated `Ikrima: Ali burnt some people and this news reached Ibn `Abbas, who said, “Had I been in his place I would not have burnt them, as the Prophet said, ‘Don’t punish (anybody) with Allah’s Punishment.’ No doubt, I would have killed them, for the Prophet said, ‘If somebody (a Muslim) discards his religion, kill him.'”(sahi bukhari 4.260).
Apparently, Ali did an ijtihad which was contrary to Sunnah, this is the reason Ibn Abbas said he wouldn't have done that, which is similar to Ali disagreeing to follow certain Ijtihad of Umar or Uthman. But this doesn't effect their Rule, as their ijtihad was legitimized per command of Prophet(saws).