  The Internet has provided us with great opportunities for large scale
collaborative public good projects. Wikipedia is a predominant example of such
projects where conflicts emerge and get resolved through bottom-up mechanisms
leading to the emergence of the largest encyclopedia in human history.
Disaccord arises whenever editors with different opinions try to produce an
article reflecting a consensual view. The debates are mainly heated by editors
with extremist views. Using a model of common value production, we show that
the consensus can only be reached if extremist groups can actively take part in
the discussion and if their views are also represented in the common outcome,
at least temporarily. We show that banning problematic editors mostly hinders
the consensus as it delays discussion and thus the whole consensus building
process. To validate the model, relevant quantities are measured both in
simulations and Wikipedia which show satisfactory agreement. We also consider
the role of direct communication between editors both in the model and in
Wikipedia data (by analysing the Wikipedia {\it talk} pages). While the model
suggests that in certain conditions there is an optimal rate of "talking" vs
"editing", it correctly predicts that in the current settings of Wikipedia,
more activity in talk pages is associated with more controversy.
