Continuity mistake: Arkham Asylum cells are different in different episodes - in Fear of Victory and Fire From Olympus they come with big windows. Whereas in others like Dreams in Darkness and Harlequinade they're smaller with small windows in the doors that are sliding open kind.
Continuity mistake: In some episodes like Trial the inmates are wearing prison uniforms while in others like Fear of Victory they're wearing their costumes. Another thing is the prison uniforms constantly change color, in some episodes they're light grey, others light blue, then in some light purple.
Answer: She escaped off-screen and created a copy of herself to fool everyone.