LOST does this a lot. It tends to cross over with Mind Screw. Notable examples:
"Walkabout": John Locke was paralyzed before crashing on the Island, but can mysteriously walk again.
"Raised by Another": It's revealed that Ethan wasn't on the plane. Then he captures Claire and Charlie.
"Exodus, Part Two": The Others confirm that they are in fact real by blowing up the boat and kidnapping Walt.
"Man of Science, Man of Faith": Someone's living in the hatch! And Jack's met him before!
"Two for the Road": Michael murders Ana Lucia and Libby, then sets "Henry" free.
"Live Together, Die Alone": Turns out that guy living in the hatch crashed the plane. And the hatch blows up. And Jack, Kate, and Sawyer are captured. And someone in the outside world learns where the island is.
"The Man from Tallahassee": Locke's dad is on the island.
"Catch-22" and "D.O.C.": Someone arrives on the island. And she says that the wreck of Flight 815 was found, with no survivors.
"Through the Looking Glass": The flashback is actually a flashforward! Some people get off the island, but they aren't happy about it. Also, a lot of people (mostly Others) get killed.
Season four finale: in "There's No Place Like Home, Part II": Ben moves the island, Michael and many of the Flight 815 survivors are blown to pieces, Locke becomes the leader of the Others, and Desmond reunites with Penny before they send the "Oceanic Six" on their way....and in the flash-forward, Locke is dead.
And the Season Five finale "The Incident: Parts One/Two" tops them all: we finally meet Jacob, and it turns out he's interacted with all the main characters at important moments in their lives. It turns out that the guy we've believed to be Jacob all this time is actually his nemesis, which carries some pretty staggering implications, especially about Jack's dad. It turns out Locke is still dead, and the man walking around alive is actually Jacob's nemesis, who manipulates Ben into killing Jacob. And in the past, the eponymous Incident occurs, so at first it seems Jack's plan to change the past has failed. Then Juliet is dragged into the Swan pit, turns out to be not quite dead, and sets off the nuke herself. Just to make it clear how totally wham this is the ending title card is flipped to black text on a white background. Nothing Is the Same Anymore. Boom.
"Sundown". Sayid kills Dogen, allowing the Monster to invade the Temple and completely eradicate all the Others who don't join him. At the end of the episode, everyone who might oppose Smokey is dead, scattered, or fleeing for their lives, and Kate can only look on helplessly as Claire, Sayid, and the surviving Others join Fake Locke.
"The Candidate". The survivors reach the plane, and decide to hijack the submarine instead due to the plane being rigged with explosives. They board and betray Un-Locke at the last minute, leaving him stranded. As soon as they think they're safe, they find the armed bomb that Un-Locke snuck into one of their backpacks. It blows up, resulting in the deaths of Sayid, Jin and Sun, while Jack, Kate, Hurley, and Sawyer escape, mourning their lost friends. Un-Locke takes notice, and heads off to "finish what he started".