"I've been watching you sleep for yeeeeeeears."
This happens constantly. An episode will end in a Scooby-Doo reversal, only to re-reverse back to the supernatural being real. In another episode, Grandpa tells the kids a story about a haunted train. The legend is debunked, but afterwards, a real ghost train is seen flying off the tracks by one of the characters.
In another, Sid is made fun of for looking like a vampire at school, and in the end, he looks at the camera and grows fangs. Everything from a headless cabbie to a prohibition gangster to a ghost bride who ax-murdered her unfaithful fiance all turn out to be real.
"Up next, Sam and Freddie can't figure out how to turn off their Snapchat filters on iCarly!"
It's even implied that one of the characters is mistakenly left to die at the hand of a ghost, because fourth-graders are cruel and unfeeling and won't give you a slice of their pizza no matter how often you tell them that you write for Cracked.com. By the next episode, no one ever mentions any of this shit. Arnold's city is full of undead murderers and monsters. And yet the most unbelievable aspect of the show is that Helga's dad makes a living selling beepers.