Wikipedia is the paradigmatic example of collaboration on the internet and its advantage is, from the research point of view, that every editing or discussion event is perfectly documented and freely downloadable. We have been interested in the emergence and resolution of conflicts. We constructed a measure, which made it possible to identify controversial articles algorithmically. We investigated the statistical properties of such pages in a number of languages and concluded that, except of a few, generally disputed topics, the most controversial pages are culture-dependent. By looking at the temporal evolution of the conflicts we were able to identify three categories: Isolated conflicts, multiple conflicts and permanent wars. We introduced an opinion dynamics model by which we were able to reproduce qualitatively our findings.