Continuity mistake: When Riddler burns his costume, he's wearing his purple gloves, but after throwing his green jacket into the fireplace, his gloves disappear entirely. (04:25:03 - 04:29:59)
Continuity mistake: Riddler's gloves disappear briefly after Batman confronts him at his party.
Other mistake: When Alfred serves Bruce coffee at the end, there's no steam coming from the coffee pot or Bruce's mug when Alfred pours the coffee into mug.
Answer: She escaped off-screen and created a copy of herself to fool everyone.