When Harley crashes the party the banner says, "HAPPY BAR MITZVAH JOSHUA" but when she's doing the bullying speech the banner says "JOSUHA" instead.
The story takes place during The Penguin's nephew's bar mitzvah, but The Penguin refers to it at one point as a "bat mitzvah," a ceremony for girls. A Jewish character would not make that mistake. However, it could be a joke--as in BAT(Man) Mitzvah.