All-Star Cast: The film featured some of the biggest stars (or at least rising stars) of the time period. Unfortunately, it's also been credited by some of those same stars for crashing their careers.
Beam Me Up, Scotty!: Mention this movie in conversation and someone will inevitably intone "Ice to see you!" in a Schwarzenegger voice. Problem is, while Arnold's Mr. Freeze does make many cold-related puns in this film, that particular line was never in Batman and Robin, and was in fact uttered by McBain, the Schwarzenegger parody character on The Simpsons.
George Clooney has made it pretty clear that he hates the film and makes many Take Thats against it; it's been rumored at various times that if you meet Clooney and tell him you saw the film, he will give you your money back. However, Clooney has also said he doesn't regret doing it at all, and in fact credits the film for making him a leading man in Hollywood.
Joel Schumacher has been apologetic about it, but hasn't really indicated whether or not he truly hates it rather than just regretting it. He mostly just sounds resentful for being singled out for blame all these years, particularly since he was specifically hired by WB to promote toys.
To the credit of the entire cast, none of them seem to regret the process of making the film; they genuinely got along, and if nothing else had fond memories of having fun working together.
Executive Meddling: Most of the decisions in the creative process were made by the marketing executives instead of the creative team.
Jesse Ventura plays an Arkham Asylum guard. Interestingly, this marked the third time that the wrestler had appeared in a film with Schwarzenegger (after Predator and The Running Man). AKA: Hey, It's That Governor!
Australians were wondering why Bruce's girlfriend is Aussie model and actress Elle MacPherson.
Gossip Gertie is played by Elizabeth Sanders, now-widow of Batman creator Bob Kane. She also appeared as the same character in Batman Forever, and also cameoed in Batman Returns.
Old Shame: Joel Schumacher apologized for this film in the DVD Commentary. George Clooney quickly distanced himself soon after it tanked. However, in more recent interviews, he's been kinder to the film, now calling it "the biggest break I ever had" since the role got him into Hollywood and paved the way for his more successful roles later in his career. He still hates the movie though. There was once a persistent, if unconfirmed, rumor that Clooney would pay back the ticket price if you told him you saw the film in theaters.
Silverstone is silent on the matter of the franchise, only chiming in to voice support for Ben Affleck's casting as Batman. She only made a couple more major film appearance after this (Excess Baggage and the better-received Blast from the Past), and it can't be a coincidence: Trouble began almost immediately when reports leaked of the actress' costume being resized multiple times due to her putting on weight during the shoot, earning the tabloid nickname "Buttgirl." Fans became nervous, and the backlash grew to the point where Schumacher himself came to Silverstone's defense in interviews, at first genially ("What is this girl's big sin - that she ate some pizza?"), but soon stopped playing Mr. Nice Guy and accused the media of promoting eating disorders in young girls. It seems everybody involved in this snafu misplayed their hand. In any event, critics were even more merciless to Silverstone when the film was released. Since then she's acted on Broadway and in a few film roles, but reportedly refuses to invite that big a spotlight ever again.
Parody Retcon: George Clooney has often claimed that he "played Batman as gay", when he clearly did nothing of the kind, presumably as a face-saving gesture for the biggest stinker of his post-ER career.
Recycled Script: Notice how most of the key plot points are nearly identical to Batman Forever, including, but not limited to: the introduction, and the final act, in which Batman, Robin, and Batgirl raided the observatory.
Star-Derailing Role: The combination of this film's failure along with Excess Baggage served to toss Alicia Silverstone out of the A-list at the height of her fame. It's also been said to have done this to Chris O'Donnell, but he says it it's not true: he had steady work offers through the 2000's but wanted to take time off to raise his kids.
What Could Have Been: Before the film was even released, plans were in motion for a fifth film (working titles included Batman Triumphant and Batman: Dark Knight). Schumacher wanted to return to the style of the Burton films, and his screenplay was supposedly Darker and Edgier than the first two films combined! The Scarecrow would've been the main villain, with Harley Quinn (here, the Joker's daughter seeking revenge) and Man-Bat (his mutation caused by Crane) in supporting roles. Jack Nicholson was in negotiations to return as the Joker (albeit as a hallucination Batman suffers caused by the fear gas).
It goes on. If they had continued after that, they could have used the Bane sequel hook near the beginning to lead up to a movie based on the Knightfall arc. And that would have meant Jean-Paul Valley... Azrael... WHY?!
In earlier drafts, Barbara wasn't actually Alfred's niece but the daughter of an old friend of his that came to see him as her "uncle". This was kept for the comic adaptation.
Barbara's Batgirl costume was to also have a full-head cowl, but Schumacher didn't like the look of it after a costume test and wanted Alicia Silverstone's hair to flow freely. Thus, the domino mask was used, though Silverstone does use a cowl as Batgirl in one scene.