Ulema العلماء (in Arabic is a plural), so if a 'Alim عالِم (singular of Ulema) or scholar with deep knowledge find out something which goes against the consensus and he is certain about his conclusion and can clearly proof it he is asked to follow his opinion!
For example if a person sees the crescent moon of Ramadan, but nobody trust him he's asked to fast!
Also read in the Quran:
And most of them follow not except assumption. Indeed, assumption avails not against the truth at all. Indeed, Allah is Knowing of what they do. (10:36)
The truth is always first to follow, so if you think something is true you are asked to follow it and the opposite of truth is assumption or falsehood.
And they have thereof no knowledge. They follow not except assumption, and indeed, assumption avails not against the truth at all. (53:28)
Knowledge (certainty) is important and again assumption is its opposite!
In osol al-fiqh there's an important intro about technical terms were terms, their meanings and their opposite are explained!