#1 Batman's favourite food is Mulligatawny soup
Mulligatawny soup, an English chicken-lamb soup based on a Tamil recipe, is Batman's favourite food, according to Alfred in Batman #701.

#2 The Beatles have appeared on a Batman cover (kind of) 
In Batman #222, writer Frank Robbins decided to poke fun at "one of the Beatles being dead" controversy. The June 1970 issue was titled "Dead...Till Proven Alive" and features Robin holding an album which is similar to the Sgt. Peppers Lonely Hearts Club Band cover. The band was called The Oliver Twist instead of the Beatles.

#3 Fans killed the Robin (Jason Todd)
This one is not a lesser known fact, but Jason Todd, at the end of Batman #427, was left to die by the Joker. The back cover listed telephonic poll instructions and 10,614 fans voted. The fate of Jason was sealed by a slim margin of 72 votes and he died in "A death in the family".

While Bill Finger and Bob Kane created the Joker and Jerry Robinson sketched it, the character was highly inspired by a photo of actor Conrad Veidt as Gwynplaine (a man with a disfigured face, giving him a perpetual grin) in the 1928 film adaptation of Victor Hugo's The Man Who Laughs.

Original sketches made by Bob Kane had the Batman donning a red costume with a bat wing-like cape, inspired by da Vinci's designs of the ornithopter (among other things)!

#6 Batman's online screen name is JonDoe297

#7 Andy Warhol made a Batman movie
In 1964, Andy Warhol produced and directed a Batman movie called Batman Dracula, without DC's permission. The movie was first to feature a campy styled Batman as a homage to the series from the bat-fan Warhol.

#8 The Arkham Asylum is real
Arkham Asylum was based on the Sanitorium in the fictional city of Arkham, Massachusetts in H.P. Lovercraft's novels, which was in turn based on the Danvers State Hospital, a mental asylum in Massachusetts.

#9 Batman hasn't always had a no killing rule
#10 Jim Gordon has a son who is a psychopathic killer; James Gordon Jr.