What's the biggest plot twist/bad news you gave your players/have received?
I'm running a Traveller game where the players escaped a planetary bombardment and clean up by making a misJump and for story reasons it deposits them a kiloparsec away in civilized space, but still a kiloparsec away; on top of that its not as simple as Jumping in a straight line, the safest route would be to follow established trade routes and that would take them 7 years.
I'm going to let them decide what to do: stick around this area and start a new life or they can attempt the 7 year journey back home. I'm also going to let the captain (a player) break the news to the rest of the crew. This revelation is going to come after the players win a speeder race and they'll still be on their winner's high.
I once ran a short little campaign of a tribe of peaceful, forest dwelling, totally hitting you over the head about Native American niceguy sterotypes, whose lands are being invaded by these tall, arrogant strangers with magic weapons and strange ships.
It was very shortly before the end of things that they realized that they were playing in Middle Earth, and they were fighting to resist the Numenoreans building a port on their turf in the Second Age.
I'm also running a Traveller game.
The group has been trying to exterminate an evil shady group who extract some kind of pain endorphin from living stumps, abducting beings and amputating all their limbs and attaching them to the "milking stalls"
Little do the group know that the pain endorphin the evil group produces is a key ingredient in the popular beverage they've been drinking all campaign.
Near-future Intersolar game, crew is sent to investigate and reboot a ship built for interstellar travel that went dark in the Kuiper Belt. After spending some time arguing with the A.I., which was given nothing to do for five years, and themselves, they realized after three fifths of a session that the ship blackout, and subsequent termination of its entire crew, defied all natural means. Suddenly, the machinist noticed the chief biologist's extra, shuddering silhouette.
High-level d&d 3.5 powergame where DM actively encouraged us to powergame and minmax. I'm playing a Wilder spamming maximized energy ray and quickened maximized energy ray, and kill the big bad boss dragon, only to find that the dragon was actually the princess we're trying to rescue, having been polymorphed and mind controlled. As it turns out, killing her ended the royal bloodline of the magical interdimensional kingdom we were running around in, and caused the universe to start falling apart.
>"You've done so much for the kingdom. You've trudged through dungeons to retrieve the ancient relics of kings past, you've slaughtered enemies to the throne, you've even married his daughter.
>"It's such a shame the king died weeks ago and you've been working for me, instead. Tragic, honestly. Guards! Seize them and divest them of their weapons. We have work to do."