When Batman and Robin drive away from the observatory, they pass by most scenery twice.
When Joker's goons attack Batman and Robin, Batman calls them "snipers", which they are not. They're ordinary gunmen.
When Robin rushes into the observatory, the eyes on his mask are blue.
Joker's last appearance in the DCAU (Justice League, "Wild Cards"), also featured a pirate television special, and in both episodes, Joker used the phrase "technical difficulties".
The Joker performs the famous kid song "Jingle Bells, Batman Smells". The song was referenced in the Justice League Unlimited episode "Panic in the Sky, when Flash uses the opening lyrics to "explain" Batman's absence.