Creator's Pest: Gerry Anderson hated John Tracy due to his "all-American" quality, among numerous other reasons he has cited over the years, which is why John is so rarely seen in the series.
Defictionalization: There's an actual International Rescue, which began operations in 1985 and named itself after the organization found in this series. That's about where it ends, though; there are (sadly) no Tracy boys and (even more sadly) no Thunderbird craft to be found. (This could also explain why IR is referred to all but once as "the Thunderbirds" in the 2004 Live-Action Adaptation.)
Demand Overload: When The BBC began re-running Thunderbirds in 1992 (after it had been off the air for several years) stores quickly sold out of Tracy Island toys. As a compromise, Blue Peter came up with a build-it-yourself version.
A positive example when Lew Grade ordered the series be made with hour-long episodes. This required the series to delve into more sophisticated plot and characters to make up for the time, which made the series a cult classic.
Sadly, a negative example from Lew Grade, and an unintentional one at that: The show was axed from airwaves after Grade's rather unsuccessful trip to America, where all three major networks were bidding on the show. Thanks in part to Grade's playing each network off the other and trying to raise the price, when one network dropped out, the others followed suit, and Grade felt that without American involvement the show was too expensive to produce.
ITV holds the rights to the series and refused to return them to Gerry Anderson or allow him to either remake or continue the series à la Captain Scarlet. By 2011, this was resolved, allowing the new series to proceed.
Fake American: All voice actors (save Virgil's, who quit after season one) were either British, Australian or Canadian, while most of the main cast was (implied to be) American. The Tracys were going to be British but then it was settled that they would be primarily American was so that the series would fair well in the U.S. because at that time the U.S. wasn't that accepting of British T.V. Sadly, the series became obscure in the U.S. and to make matters worse, the most attention anything related to the name of "Thunderbirds" was the 2004 film.
Six episodes were edited into a trio of compilation films for American broadcast in the early 1980s. While the remaining episodes were later released with no major edits, the only way to see the outstanding six for a considerable period were to tape them during the 1990's reruns, or import copies from countries where the episodes were released unedited (such as Australia).
After securing the rights to the series in 1999, Carlton hastily remastered and released Thunderbirds on DVD and VHS. Unfortunately, while the films were replaced with their original counterparts, every episode featured new (and sometimes incredibly loud) sound effects. In the case of "Trapped In The Sky" and "The Perils Of Penelope", both were given new title cards.
Even the Blu-Ray releases weren't immune to pruning. In a bizarre attempt to sell the series in widescreen, ITV cut off the top and bottom of the fullscreen prints. The North American Shout! Factory releases do not do this.
The Other Darrin: Virgil was voiced by David Holliday in Series 1 and Jeremy Wilkin afterwards.
Screwed by the Network: According to ITC themselves, Fox Kids' hacking to bits of the show was the reason why it failed and was taken off the schedule after about a month. (Still didn't stop them from creating the Turbocharged version.)
Screwed by the Lawyers: Apparently, the reason why the Fox Kids and Turbocharged version were redubbed was because Michael Jackson had the rights to the original music and vocal tracks, having purchased them in the mid 80s. (You might be wondering how that happened. When Jackson purchased the rights to The Beatles' catalog, it included the rest of ATV Music-ATV having been Lew Grade's ITV station in London and the Midlands. ATV and ITC were fused at the hip during the era, and hence presumably it came under ATV's music dept. which then got bought by Jackson.) However, it seems all that legal stuff got cleared up by the late 90s and all further releases have had the original music/voice tracks (though several sound effects are still dubbed over, but not in every episode).
Short-Lived Big Impact: The series was cut short after just 32 episodes, yet it managed to retain a loyal fanbase to this very day, is still regarded as Gerry Anderson's best work, and still has other shows pay tribute to it.
Technology Marches On: A given in the Gerry Anderson shows. You have space stations, hypersonic rocket planes, and a mobile computer the size of a grand piano!
Throw It In!: During filming for the episode "Trapped in the Sky", Elevator Car No. 3 went out of control. The production team decided to include this in the episode, adding a shot of it crashing into a nearby airliner.
Unintentional Period Piece: Given this was made in the '60s, this was going to be inevitable. Modern British viewers may be a little miffed at the main airport being called "London Airport", unaware that back in the day, this was actually the name for Heathrow Airport before Stansted and Gatwick acquired "London" status. There are also several references to Cape Kennedy.
Gerry Anderson and later on, whoever happened to own the rights to the original series, had tried multiple times to get the series back on the air; some of the later Anderson-backed projects were rewritten to remove references to this show, but then got cancelled during production for one reason or another.
John was originally conceived to be the pilot of Thunderbird 2.
Fenella Fielding was the original choice to voice Lady Penelope.
Several shots of Thunderbird 3 in action are blatant references to Bill Paxton's Apollo 13, as admitted by Jonathan Frakes on the DVD commentary.
A Director Allusion- when Alan goes to pilot Thunderbird 4, he tells Fermat "You have the bridge" of Thunderbird 2- that being a frequent line directed towards Commander Riker in Star Trek: The Next Generation.
Creator Backlash: The original series' creator Gerry Anderson, who only saw it on DVD, initially disliked the film, calling it the "biggest load of crap I've seen in my entire life". However, co-creator Sylvia Anderson had a more positive reaction.
An alternate version of Thunderbird 2 landing in London.
FAB-1 being torpedoed by The Hood's submarine, leading to Penelope and Parker having to use a pedal-powered boat to reach the island.
Disowned Adaptation: Gerry Anderson himself is on record as saying that Team America: World Police — a movie he didn't entirely approve of — was closer in spirit to the original show. Helps the studio didn't even hire him to be a creative consultant, feeling there were too many people in the creative team.
Those Two Actors: Brady Corbet and Vanessa Hudgens had been in another film from Working Title the year before, 13.
Troubled Production: Had been in Development Hell since the mid 90s. It was originally conceived as a CGI film, inspired by the success of Toy Story and then later as a vehicle for the Baldwin brothers (with Sean Connery as Jeff). Gerry Anderson was brought on as a creative consultant but let go when Working Title felt that they had enough people on the creative team already. Eventually it entered production with a much younger target audience in mind. Three major script changes also happened during production; not helping was the film "falling through the cracks" when Seagram (the liquor company) acquired Polygram Filmed Entertainment (who had started the project during their ownership of the ITC library) and merged it with Universal. Jonathan Frakes did manage to knock $3 million off the budget due to his fast shooting style, but that seems to open up another can of worms altogether; outside of the death of a rigger who fell while dismantling a set, and a few cast members getting heatstroke, the filming itself went smoothly.
Uncredited Role: The dialogue between Lady Penelope and Parker was written by Richard Curtis which helps explain why their scenes were one of the more praised aspects of the film.
In 1997, a totally different movie was in the works, with Peter Hewitt directing. The tentative cast was Johnathon Schaech and Tom Everett Scott as Scott and Virgil, Matt Damon as Alan, Steve Zahn as Gordon, Rowan Atkinson (!) as Brains, Liv Tyler as Tin-Tin, Kristin Scott Thomas as Lady Penelope, and Pete Postlethwaite as Parker; it's unknown who would've been Jeff, but rumor states that Robert Redford was being looked at. The plot would've revolved around IR being targeted by deranged Moonoleum Incorporated director Thaddeus Stone, who had constructed an orbiting atmospheric extraction device that would transfer Earth's atmosphere to Stone's colony on the Moon, leaving the Earth barren and lifeless. In order to complete his plans, Stone needed to get his hands on Tracy Island's power core, the only generator powerful enough to fuel his extraction device. With assistance from The Hood, Stone learned the location of Tracy Island, kidnapped Alan and forced Jeff to hand over the power core, leading to a race against time to save the Earth- amongst the details were a new rendition of the Skythrust airliner (which would've crashed into Big Ben), a sports car named after the Fireflash and the dramatic self-destruction of Thunderbird 1 after a high speed night pursuit over Tokyo.
You know that bit where Parker cheers that England's national football team have won, causing him to be distracted from flying FAB 1? Well, he was originally supposed to cheer on a reported victory of Tottenham Hotspur- they chose that due to the clubs proximity to Wormwood Scrubs prison, Parkers former "accommodation"- but ultimately it was changed to avoid alienating parts of the British audience (that was bound to happen anyway).
The Tracys were supposed to wear color-coded baseball caps early on, and some were even made, but they didn't go over well in screen tests and were subsequently dropped.
They had initially attempted to have Rolls-Royce build a new FAB-1, but BMW (RR Motor Car's owners) said no because they were busy with creating the then-new Rolls-Royce Phantom. Ford took the job instead.
An idea for the climax was to have the Mole as piloted by The Hood sever one of the supports of the London Eye and one of its passenger pods would fall in. However, the people who ran it said no, on account of it being a potential terrorist target, and hence was changed to the "Olympic Monorail". Another one was that Thunderbird 2 would have swept through other parts of London, including near the Houses of Parliament and also past the Mayors building, blowing all of its windows out. This was also dropped for security reasons.
There was talk about having Sylvia Anderson and Frakes as passengers on the monorail, but scheduling issues prevented Anderson's appearance; Frakes instead cameoed as a cop at the end.
Promoted Fanboy: Composer Ed Potokar was a fan of the original Thunderbirds as a kid.