I don't know if this has been posted before, but I have just finished watching The Dark Knight Rises again and noticed this little detail. If you look at the boy wearing glasses he has a Harley Quinn costume on his mitten.
Sure, there have been plenty of thoughts about Joker making an appearance, or at least being mentioned or teased, in the end Chris Nolan decided to honor Heath Ledger's performance by not letting any Joker references into the movie. We can only hope that we will see Harley in future installments.
It's not much of a debut for Harley on the big screen, but it's always nice to see small treats like this