In the end credits of every episode of It's Always Sunny in Philadelphia the credits include a hidden audio message. The messages are recorded by Mac, Charlie, and Glenn and are played backwards. When played forward, the audio message for the first season says, "You're stupid for playing this forward."
There's a pineapple hidden in every single episode of Psych. Sometimes it's an actual pineapple, but sometimes they'll sneak one in a on shirt pattern or a keychain dangling from a door caught on security cam footage.
Walt took of his pants the first time he and Jesse cooked meth in the To'hajiilee desert. Towards the end of the series, Walt travels through the same location with his barrel of cash. Look at the bottom left corner ”” appears his pants didn't get up and walk away!