It's bad. Very, very bad. It a series of flights that for whatever reason has been determined by a select group of pilots, to be a hazard, due to a number of pre-described criteria, such as a flight that historically always operates over some limit, but the company builds it showing that it's block time is under that limit. Day/night cycles with minimum rest, into or out of busy airports, very long duty days, made longer by flying in the middle of the night to a station then sitting there for a few hours, then flying another leg into a congested area, during rush hour. All of these could cause a series of flights to become disputed.