Ibn Taymiyyah is a very noble scholar and yes, he contradicted ijmaah but later renewed his opinion (things like tawassul etc). He also wrote masterpieces venerating our beloved peace be upon him yet why is he still known as ‘daal mudhil’?
There are certain issues by which Ibn Taymiya went against the doctrines of Ahles Sunnah Wal Jam’ah. He deviated from the mainstream Muslims in many issues of Usool and Furu’. Such as:
Ibn Taymiyah`s saying of Hawadith with no beginning existing eternally with Allah. His saying of the sitting of Allah, Ta’ala, prohibiting the Tawassul by the Anbiya and Awliya and the Tabarruk by them and their traces and his disagreement with the Ijma of the Muslims in the divorce issue.