The concept is dramatized in the (fictional) soap opera within the movie Tootsie, where the (female) character played by Dustin Hoffman reveals that she is a man, thus completely changing the continuity and forcing the fans to reevaluate earlier episodes.
George Lucas is known for making his movies very family friendly, and both Episode I and II have serious moments but are relatively light. Episode III features Anakin slaughtering Jedi "younglings" heartlessly, which is so vicious that it probably caught everyone off guard.
The Matrix: Reloaded ends with Neo's meeting with The Architect, sealing mankind's fate by choosing to save Trinity, and somehow demonstrating his powers in the real world. Oh and Smith is in the real world too. The entire second half of Revolutions contains much more wham however.
Back to the Future Part II ends with Doc Brown and the DeLorean being struck by lightning and being warped to who-knows-when. This leaves Marty stuck in the past, and the past Doc Brown is his only hope of finding the other Doc and getting back to the future.
The Dark Knight: Rachel is killed. Later Harvey goes off the deep end and becomes Two-Face, Batman ends up killing him, and takes the blame for all of his crimes to ensure that Harvey Dents reputation isn't destroyed and all the criminals he prosecuted stay in prison.
How can you tell a Firefly fan hasn't seen Serenity? They smile at Wash's "I am a leaf on the wind" line from the trailer, not knowing what comes after his final, victorious statement.
The whole film goes from wham moment to wham moment... it even opens with a wham moment: Simon is the one who did the physical infiltration and rescue of the academy to get his sister out? Suddenly, everything we know about him from the series has been turned on its head (but it does give us a new insight into the episode "Ariel"). Then we learn the government made the Reavers. Then Book dies and the Operative kills anyone who's ever sheltered the crew just in case Mal decides to go into hiding with any of them. Then Mal turns Serenity to a Reaver ship, even putting several bodies of friends they'd once known onto the ship to add to the vision. Then the above mentioned wham moment kicks in.
Transformers: Dark of the Moon: Not only does Sentinel Prime betray the Autobots by striking a deal with the Decepticons, and Dylan Gould, Carly's boss, is revealed to be working with them, but also Cybertron is presumably annihilated by the end, making Earth their only remaining home.
The Avengers is this for the entire Marvel Cinematic Universe: The rest of the universe now know Earth is not as defenseless as they once assumed, with Thanos now wanting to "court Death" by challenging Earth's mightiest heroes somewhere down the line.