Epilogues are almost always a sign that you don't think you proved your case.
That's my categorical judgment. What do you all think?
I'm talking about the sort of epilogue that is not a twist (like the ironic epilogue in Indiana Jones, where we see that the Ark that Indy and the Nazis were fighting to the death over is going to lie forever forgotten in some government warehouse) or a poignant bittersweet moment of completion (like in the final Prison Break, where the conspirators all gather, and only Michael is missing, and we realize it's for the dedication of his tombstone, and he has died). I'm talking about an epilogue that is really about reassuring the reader that yes, this couple ended up married and pregnant, so of course they love each other; or the one the detective outed as the murderer is convicted, so see, he must have really done it! That is, it's giving the reassurance that the story question was answered appropriately... however, it indicates that you think the reader NEEDS that reassurance. Why? Because you don't think your actual story events and resolution scene did the job.
Now, see, I think prologues often have their uses, but I think epilogues are seldom necessary (except maybe to provide a final twist), and if the epilogue IS necessary, I would take another look at the final scene before that-- what was insufficient about that, and can that be fixed?
So what do you think? What do you think would be an effective epilogue, and when do you think an epilogue should be extraneous? What works and what doesn't? Have you ever written an epilogue, and why?