Episode: Fear of a Bot Planet

During the trial in which Fry and Leela are falsely sentenced to serve hard labor, they are confined to a cage with their hands and feet cuffed together. They are then forcibly dropped into a dark chamber below. When the lights are turned on, the shackles around their feet have now disappeared, whereas the handcuffs remain in place.

Later on when the elder bots lurch forward to execute Fry and Leela, they stand up straight in surprise, at which point the shackles have now reappeared around their feet.