As with all functioning society, a set of moral rules and conduct is in place to maintain common civility. Fly Fishing is no exception to the imposition of a moral code governing its rules and ethics. Various flavors of this code book have been authored from time to time leaving many confused as to the rights and wrongs of the sport.
I would like to include a long forgotten but most important commandment: Never fish too close or violate the right of way of another fisherman. It is always first come first serve out there. To maintain civility between us all, you should always ask permission before entering another anglers beat, typically delineated by obvious structural changes in the water between riffles and pools. Otherwise you will probably get punched you in the eye or verbally humiliated in front of your wife and kids.
Read about the "Seven Deadly Sins of Fly Fishing" here.