That is why in Quran 4:59, we find that people were asked to refer the infallible sources in regards to any such dispute with the one in authority/leader. And the infallible sources are Allah and Prophet(saws). The rest who are in authority are all fallible.
Then what is the use of Ijma and Qiyas (& Ijtihaad)?
*Ijma and Qiyas as well as Ijtihaad are used when one cannot find the solution of the problem(s) being faced in Al-Quran and Sunnah of Prophet (s.a.w.w)
It is possible to interpret Al-Quran and Hadiths according to one's own desires. Infallible sources would be interpreted by fallible people and therefore disagreements and disputes would still exist.
Making Ijtihaad is a part of Sunnah, it won't be considered, changing the Shariah, even if the Mujtahid erred.
Amr ibn Al-As reported: The Messenger of Allah, peace and blessings be upon him, said, “If a judge makes a ruling, striving to apply his reasoning (ijtihad) and he is correct, then he will have two rewards. If a judge makes a ruling, striving to apply his reasoning and he is mistaken, then he will have one reward.” [Source: Ṣaḥīḥ al-Bukhārī 6919, Ṣaḥīḥ Muslim 1716]
And there weren't any other infallible people after Prophet(saws) to make ijtihaad.
عن علي قال : قلت : يا رسول الله ، إن نزل بنا أمر ليس فيه بيان : أمر ولا نهي ، فما تأمرنا ؟ قال : تشاورون الفقهاء والعابدين ، ولا تمضوا فيه رأي خاصة
Ali(ra) said: Oh Messenger of Allah Salallahu `alayhi wassalam: If a matter comes upon us about which there is no information, neither to do it nor to abstain from it. so, What do you command us regarding it? He (the Prophet) said consult your (righteouss) Fuqahaa and (knowledgable) Aabideen(worshipers) and Do not execute (orders) on the opinion of any (one) specific person. [Al Mu`jam al Awsat, Hadeeth 1641, Classed as Hasan by Abu Yahya noorpuri]