Another unusual element of the category is the voting process, which was adjusted in 2012 after a committee led by Michael Moore (a member of the Academy's governing board), petitioned to change them. Previously, the nominated films were chosen by small sub-committees, meaning that just one or two people could block a particular film from a nomination.
With the new rule change, the entire documentary branch is involved in the selection, making it less likely that just one or two people could change the outcome. (Moore explains the changes here.) Still, there are only 160 members of the documentary branch -- a relatively small number, compared to the more than 6,000 total academy members voting in other categories -- and there is still the possibility that if some voters hate a film, they could essentially ruin its chances of being nominated, as Jason Bailey points out at Flavorwire.
Some of the most surprising snubs in the category in recent years have been "Gimme Shelter" (1970), Errol Morris' "The Thin Blue Line" (1988), Moore's "Roger & Me" (1989), Werner Herzog's "Grizzly Man" (2005), "Last Train Home" (2010) and Herzog's "Cave of Forgotten Dreams" (2010).