Keep Circulating the Tapes: No longer in effect, as Warner Bros. , through their Warner Archive DVD-on-Demand program, released SWAT Kats in December 2010, with some missing scenes restored (the title card for "The Pastmaster Always Rings Twice", and the farmer getting devoured in "The Giant Bacteria"; although in January 2012 they issued an updated version, mainly because the original DVDs had the ending credits wrong for many of the episodes.
The Other Darrin: A few characters had different voice actors between seasons, and in some cases, between episodes.
The Sergeant was first voiced by Ed Gilbert for most of Season 1. Beginning with "Enter the Madkat," however, he was voiced by Jim Cummings. Cummings voiced him for the remainder of Season 1 and all of Season 2.
In "The Wrath of Dark Kat," Al the Kat's Eye News helicopter pilot is voiced by Frank Welker. The next time he appears, in "Night of the Dark Kat," he's voiced by Rob Paulsen.
Dr. Greenbox is voiced by Robert Patrick in Season 1's "Chaos in Crystal." By the time he reappears in Season 2's "Unlikely Alloys," Nick Chinlund has taken over the role.
Shrug of God: Whether or not Dr. Zyme really was Killed Off for Real in "The Giant Bacteria". The writers, when asked, tend to avoid answering the question or give answers such as Glenn Leopold calling Zyme "survivally challenged" instead of dead. The closest to a confirmation comes from Christian Tremblay, who said, "He probably died!"
Jim Cummings whenever Dr. Konway and Mayor Manx were onscreen together in "Mutation City". This probably cropped up a lot more on a smaller scale, as Cummings seemed to voice a solid third of the one-shot and background characters.
Unfinished Episode: The series was canceled so abruptly that three episodes were left unfinished; they were storyboarded and the voice tracks were recorded, but they were never animated. More information (but not much) is available here.
What Could Have Been: According to Lance Falk, a writer for the show, "When Strikes Mutilor" was supposed to end with the pacifist astronauts being human; with two of them removing their helmets and commenting how there's intelligent life, and we see an American flag in the background, confirming that the series took place on another planet in the future. However, H-B exec Buzz Potemkin "didn't get it" and had them changed to kat aliens, much to Falk's displeasure.
Lance Falk had big plans for Commander Feral in his initial pitch for "Cry Turmoil". It was originally Feral, not T-Bone, who joined Turmoil's forces. Unlike T-Bone's Fake Defector act in the finished episode, Feral's Face-Heel Turn would have been real—until he realized that the SWAT Kats were going to win, which prompted him to betray Turmoil, then claim he was planning to do so all along. In the end, only Feral and the audience would have known about his trip through the Face Heel Revolving Door.
Turmoil was planned to return in one of the Unfinished Episodes, "Turmoil II: The Revenge". She escapes from a women's prison, taking all the inmates with her, and gains control of a Kill Sat. The SWAT Kats go after her—but first they have to infiltrate her fortress, "a castle on top of a snowbound mountain". Also see Commander Feral's entry.
Lance Falk had two unrealized ideas for future appearances by Rex Shard, the villain of "Chaos in Crystal". Each time he returned, he would get a different power to use against the SWAT Kats (what would have been his second appearance would have him gain power over the weather by diving into an experimental energy source, and freezing Megakat City). And each time the pilots defeated him, he'd show the effects of the previous encounter. For example:
Falk: "He’d lose an eye, so from then on when you saw the guy he had an eyepatch. And each time he comes back, he’s a little more beat up. He’s become a fanatic, and he’s lost all sight of everything but revenge, and it’s really destroying him.”