The impossibility of agreement on what should be allowed
danluu.com
danluu.com
I once talked with a judge who told me "I try my best to be just, but when it's impossible to be just, I have to settle for arbitrary". When refereeing sport, I've found that a very useful philosophy: most of the time when you blow the whistle, there was a clear foul, but sometimes although it's obvious someone fouled it isn't obvious who did. In those cases I just do my best to break the marginal calls evenly for both teams.
(also, as a ref we'd have regular meetings, going over the most contentious plays and coming to rough consensus, in an attempt to ensure that we were collectively providing consistent calls during the season, even if the "ref line" wasn't necessarily what any of us would have had as a personal interpretation of the rules that season)