Bruce Wayne/Batman has appeared on every show in the DC Animated Universe, but this is the only crossover episode in which characters from another show (in this case Superman: The Animated Series (1996)) appear on his.
Batman and Superman discovered each other's secret identities in the Superman: The Animated Series (1996) episode World's Finest (1997).
Batgirl/Barbara Gordon and Supergirl/Kara Kent almost instantly hit it off as friends, which is the complete opposite of Batman and Superman's first encounter in the Superman: The Animated Series (1996) episode World's Finest (1997).
When Batgirl escapes from the Batmobile, her theme from Batman: The Animated Series (1992) plays as she flies through the air.
Penguin makes a snaky comment that Kara is too young to enter the Iceberg Lounge, while Barbara was admitted to the Lounge in Joker's Millions (1998). According to her hospital file in Batman & Mr. Freeze: SubZero (1997), Barbara was 20 at the time of that film. However, since Kara was placed in cryonic stasis as a teenager shortly after Krypton exploded, as seen in the Superman: The Animated Series (1996) episode Little Girl Lost (1998), making her chronologically much older than Superman/Clark Kent. However, Kara is later seen officially turning 21 in the Justice League Unlimited (2004) episode Far from Home (2006).