The Sponge Who Could Fly (aka The Lost Episode) - S3-E16
Corrected entry: At the end, Patchy the Pirate pushes the wrong button on the remote, and the tape starts shooting out at him. One of the next shots shows him squirming around in the tape, and then he rolls over onto his stomach. He kicks his legs in the air, and both of his feet are visible, but earlier in the episode, when he was watching the lost episode, one of his legs ended at the knee.
Correction: Remember this is Spongebob - Patchy's eye patch changes eyes various times, and remember at the beginning of the episode, he puts a fake leg on the leg that ends at the knee. Who's to say he didn't put it on whilst he was watching the episode?