I was under the assumption that any mission you abandon can always be reactivated at a later time. I did this with the "Traitor" mission (Imperial, Sith Marauder) on Alderaan because part of the mission was a Heroic (correct me if I'm mistaken) and thought I wouldn't be able to complete it. Now I find that I can't do the mission anymore; the NPC (Lord Jorad Thul) won't grant it.
So, is it a bad idea to abandon missions, or is this just an isolated case/bug?