On the episode where Bart and Lisa join hockey teams, watch where Homer congratulates Bart after a game. He gives him a turtle for it, yet, if you watch (not closely even), you'll notice Bart takes the turtle, and puts his arm around Homer or something, basically, the turtle dissapears. WHERE'S THE TURTLE?!?
It's a turtle they had to put in with CGI so it was never real. If they did put a real turtle in the frame PETA would have sued for animal cruelty and so I think the staff just wanted to save the headache and CGI it so they didn't have any lawsuits on their hands. It's all politics.