Following its release, both Monica Keena and Katharine Isabelle had rather choice words about the film, with Keena in particular saying that she thought the screenplay was terrible, and that she only really signed on for the money and to raise her profile. Isabelle's problems were mostly to do with the fact that she didn't get along with director Ronny Yu, who tried to force her to do a nude scene even though she had specifically requested a "no nudity" clause when she signed on.
Averted by Kelly Rowland. While several journalists and television hosts have tried to embarrass her over her role in this film, she has always responded by saying that the film was a hit and she enjoyed making it, and therefore has nothing to be ashamed about.
Development Hell: Back when the two franchises were owned by different studios, the project was held up because Paramount (who owned Friday the 13th) and New Line Cinema (who owned A Nightmare on Elm Street) each wanted to license the other's character and make the film their way. After the sale of the Friday rights to New Line, the project sat in Development Hell for years as New Line went through several different scripts. The studio eventually hired writers Damian Shannon and Mark Swift, who produced the script that was turned into the film by throwing out every other script before theirs and working from a set of rules that respected both franchises. The whole story - and it's a doozy, especially when you read about the many different scripts New Line went through - is a special feature on the DVD of the film.
The Other Darrin: Ken Kirzinger was used as Jason instead of Kane Hodder because his eyes were softer and more sympathetic, allowing the audience to have more empathy for Jason.
Word of God also stated that he wanted an actor that could tower over Freddy.
Pass the Popcorn: The ads for the film described Freddy and Jason as if they were two famed wrestling champions about to face off in the ring. Before the film was released, there was an actual "weigh-in" ceremony in Las Vegas featuring the two slashers, done in the style of boxing and MMA weigh-ins. They even got Michael Buffer for it!
There are no less than four unused scripts floating around the Internet. Such scrapped plot elements included: Jason driving a Subaru, Jason helping the main characters, the final fight between the two taking place in a burning mall, Freddy killing a girl by turning her into a giant lobster and flash-steaming her, Jason killing a man via shotgun Ass Shove, a Freddy-worshipping cult called the Fred Heads, lizard-people living in the Dream World, Jason being bought back to life with the heart of the Final Girl's dead boyfriend (also coincidentally named Jason!), and Freddy summoning Charles Manson, Ted Bundy, and John Wayne Gacey to aid him in the fight against Jason. Said unused scripts can be read (in PDF form) here.
In one of the original endings Freddy and Jason continue their battle in hell, when suddenly a figure steps out of the shadows and separates them. Who is it? Why, none other than Pinhead, of course, who calmly asks, "What seems to be the problem, gentlemen?" The nightmare of licensing issues between New Line and Paramount were enough without bringing in yet another studio's character and thus, preventing this awesomeness.
One planned sequel would have brought Ash into the action, but Sam Raimi ultimately nixed it - apparently just in case he ever gets around to making the long-awaited fourth movie. (The script was used to make the first comic mini-series.) There was also talk of bringing Michael Myers in after that.