In the Bat-man Beyond episode, "Where's Terry?", Terry (Bat-man) has to stop Shriek, (the guy that has sound blasters for weapons) he is in an old train tunnel. He befriends a kid who's name is Dak. Dak tries to get Terry out of a cave in, but gets captured by Shriek. In the episode, "Shriek", Skriek loses his hearing and needs a hearing aid, so in this episode, he still has it. He is arguing with Dak, the boy, to tell him where Bat-man(Terry), is. But the whole time he is arguing with Dak, he is not wearing his hearing aid. he has it on earlier in the episode when he is talking to his assistant. I don't know why he wasn't wearing it, but it's sure a slip up.