When Wikipedia editors disagree about acceptable content (week 12), and when the B-R-D cycle (week 13) breaks down, the result can be an unpleasant exchange of sniping, hostile edits, and hasty undos. Wikipedians call this edit warring.

There are other causes, too: as I wrote in Quartz this week, sometimes edit wars occur when an offsite mob (sometimes fans of Beyoncé or maybe Gamergate culture warriors) overwhelms editors by making unacceptable changes over and over (sometimes for the lulz, just to vent, or to score ideological points).

Wikipedia maintains a humorous page called "Lamest edit wars" identifying some memorable fights in the site's history. The most famous concerned what to call the city named Gdansk in Polish, and Danzig in German. If you want to see how that was resolved, check out Andrew Lih's book The Wikipedia Revolution.