Biggest Plot Twists in Cartoon History (SPOILER VER.)

What is the biggest plot twist/reveal in Cartoon History based on the cartoons you've watched? This thread will be the spoiler version, so I will make another thread where you just list the cartoon title when I get the chance. Movies and shorts are allowed. Non animated shows that have animated segments are not allowed unless they reveal the plot twists/reveals. If you can remember the season and episode, you can choose to list it here (don't list the season or episode in the non-spoiler thread to spoil when the twist/reveal will appear). If this gains enough traction I may make live action or anime plot twist/reveal threads. 

P.S. Don't just look up a plot twist/reveal. Your answer must be from previous knowledge.