Bart is worried that Sideshow Bob is out to get him:
Homer: You're the one who ruined all of Sideshow Bob's criminal schemes.
Marge: We're very proud of you, by the way.
Homer: And sure, he's probably so insane with rage that he'd butcher you horribly if he could.
Marge: But he's safely locked away.
Homer: In a minimum security prison.
Marge: For life.
Homer: Unless he gets out somehow.
Marge: Which is impossible.
Homer: Or so you'd think, except he's done it so many times before...