If you make a sudden move to the side, you could kick his bicycle out from under him. If the rider in front of you does this to you and your wheels touch, steer into him hard - the worst that can happen to him is that you displace his wheel, but the alternative is crashing and having the rest of the group run over you. If you are in front, use hand signs. Otherwise, immediately duplicate the hand signs of the rider in front of you even if you can't see the obstacle yet. Remember, they usually can't see much of the road in front of them.
Railroad tracks are dangerous.
Never brake without announcing it first. Remember that when you are in the front position you take responsibility for the entire group. You are their eyes, and you have to think ahead for all of them. For example, don't blast through yellow lights because the last riders will be forced to run a red light. Before getting seriously tired in the front position, accelerate, move to the side, and wave the following riders through.