When Tod is being chased by Chief a second time (as an adult), there are several shots where his collar is missing.
During the first chase, Chief goes after Tod while dragging his doghouse barrel behind him. When Amos runs out after them in the next scene, however, there are still two barrels: one with Copper tied to it and an empty one that should still be attached to Chief.
At the beginning of the film, just before we see Tod's mother, the camera zooms through a spider's web, however the shot is very blurry and out of focus. According to some of the animators who worked on the film, the camera operator for that scene was new and didn't know how to properly film the animation of the spider web before the shot of Tod and his mother.
Creator Backlash: Several notable animators, including John Lasseter, Don Bluth and Tim Burton, rarely speak kindly of this film, citing its tight-budgeted animation, which all but did away with the innovative technology the company had invented, as the final sign that Disney had become a shell of its former self. Bluth, in particular, took it the hardest by leading a walk-out of several other animators who followed him to work on The Secret Of NIMH during this film's production, beginning a long and bitter rivalry between him and the studio which went on until he retired in 2000.
Creator Breakdown: Tim Burton literally went insane trying and failing to replicate the Disney style. Highlights from his tenure include locking himself in his closet, biting people who came near his desk and wandering the halls after having his wisdom teeth pulled and letting his mouth bleed all over the floor. He kicked around the studio for two years, doing odd jobs for them until he was fired for "wasting" the studio's money on his live-action short Frankenweenie. They eventually made up, though.
Tim Burton: I worked for a great animator, Glen Keane. He was nice, he was really good to me, he's a really strong animator and he helped me. But he also tortured me because I got all the cute fox scenes to draw, and I couldn't draw all those four-legged Disney foxes. I just couldn't do it. I couldn't even fake the Disney style. Mine looked like road kills... imagine drawing a cute fox with Sandy Duncan's voice for three years. It's not something that you can relate to very much.
Dueling Movies: An unintentional example: the movie was released in the USA the same year when Vuk the Little Fox was released in Hungary. Both movies star a fox, who is pursued by a hunter and his dogs. Since very little information went through the Iron Curtain about animated films in production, it is unlikely that the creators of either movie heard of the other project.
End of an Age: The film stands at an overlap between an old Disney and a new one: it was the last film to have any involvement from Disney's Nine Old Men, the remainder of whom at the time retired during production (apart from Eric Larson, who remained as a trainer and consultant until 1986; he then died in 88), as well as one of the last productions to be handled by Walt Disney's descendants. However, it was also the start of a new era, as the influx of fresh-faced artistic talent (save John Lasseter and Tim Burton, who's nasty experience of the then in-the-pits studio resulted in their being let go), along with the new management team of Eisner, Katzenberg and Wells, would spend the next decade making the films which eventually became the Disney Renaissance.
Recursive Adaptation: The Disney books were inspired by the movie, which was inspired by the original novel.
Troubled Production: There were many troubles going on with the production. Several veteran animators either retired or died early in production, batches of animation drawings were stolen, leaving several scenes to be rotoscoped from pencil tests, and Don Bluth led an exodus of practically half the animation team, which delayed its release by six months and turned him into Disney's Arch-Enemy for a long while.
Don Bluth was assigned to co-direct the film until he and his colleagues departed from the studio in 1979. Scenes he worked on were the scenes of Widow Tweed milking the cow and her grabbing Slade's gun and accidentally shooting it.
Originally, Chief was going to die, thus making Copper's revenge against Tod much more extreme, but the idea was discarded for being too dark and was rewritten so Chief would get injured instead.
Keep Circulating the Tapes: It's pretty hard to come across a copy of the book, which has been out of print for decades. If you want to buy one be prepared to spare $200 to $300.
While a physical copy might bankrupt you, there is now an e-book version available.
Science Marches On: Canines like dogs and foxes are not, as it turns out, completely colourblind, only red-green colourblind as compared to the average human.
This was the first movie to feature a lot of the animators who would go on to play pivotal roles in the Disney Renaissance movies and the studios that sprang out of them such as Pixar and DreamWorks, which ironically crushed Bluth in their wake.
This was the final movie that the whole of Disney's Nine Old Men worked on; all of them retired around this film's release, with Eric Larson staying on as a trainer and supervisor. Wolfgang Reithermann, who was the effective head of Disney Animation, died in a car crash around The Black Cauldron's release, and his death plus spiritual successor Jeffrey Katzenberg getting the department dropped in his lap PLUS the massive failure of Cauldron spelled the end of the old style of animation moviemaking.