The Mole is missing from the end credits
Ray Barrett performs Alan's single line of dialogue as Matt Zimmerman, had yet to be cast
The TX 204 aircraft later appears in The Mighty Atom (evacuating the people at the Melbourne Power Plant) and Edge of Impact

Scott's mobile camera is used again in Edge of Impact

This is the first episode to feature Brains' robot Braiman, who goes on to make guest cameos in Edge of Impact and Cham-Cham but does not play an active role in either

This is Lady Penelope's first mission and visit to Tracy Island since International Rescue began operating
This is the only episode to feature all the main characters and all 5 Thunderbird craft
The opening of this episode takes place a year before International Rescue began operating. This is mentioned in the story's dialogue. If the radioactive cloud cleared away on Monday, October 6^th 2064, it began on Monday, September 29^th 2064

Ray Barrett's voice of Lovegrove is based on distinguished actor Sir John Gielgud
This is the only episode to feature (part of) a human face when Lady Penelope puts her eye against a keyhole

This is a sequel to the first episode Trapped in the Sky. Events to that episode are mentioned by a TV reporter who previously appeared in Sun Probe
The original script shows that the working title for this story was The Test Crew
Lt. Burroughs previously appeared as Solarnaut Asher in Sun Probe. The EJ2 Jet which the saboteur escapes in is later seen as the fake Thunderbird 2 in The Impostors

Grandma leaves her home to live on Tracy Island. Unless she was visiting before, then this episode must precede previous episodes (including Sun Probe, The Uninvited, The Mighty Atom, Vault of Death and Operation Crash-Dive)
Virgil's painting of Alan later turns up at the art gallery in The Duchess Assignment

Up until this point, the Hood has not been referred to by any name, not even 'the Hood'. When he calls General Strond, his codename is Agent 79 (Seven-Nine). His price for the secrets of International Rescue is $200 million

It's the only time we see Scott piloting Thunderbird 1 in civilian clothing
The monorail also appears as the Pacific-Atlantic Monorail in Brink of Disaster

Among the audience at the Ned Cook show at the end of the episode are Doctor Godber (The Perils of Penelope) and producer and agent Bletcher / Maxies (Martian Invasion / Cham-Cham)

When Eddie jumps from the roadbuilding tractor it is actually the Scott puppet that is seen landing on the ground

Dr. Korda must have the same taste as The Hood, as the statue that appears in his office also appears in The Hood's temple
The music used for the part where the MSP crosses the bridge is called March of the Oysters and was originally composed by Barry Gray for the Stingray episode Secret of the Giant Oyster. It also crops up in 30 Minutes After Noon, The Impostors, Cham-Cham and Security Hazard (this episode's flashback)

When Jeff says 'Tin-Tin's boyfriends', he is referring to Eddie Houseman from End of the Road, which was filmed before but transmitted after Edge of Impact in the original broadcast order

This episode was one of the rare cases that Alan did not appear at all. John was at home so that puts Alan in space

There is a use of a hand in the same frame as a puppet. In the castle, a human hand (Southern's) can be seen playing with a pen while the puppets of Kenyon and Dempsey can be seen in the background

This is the only episode out of all 32 episodes and both movies not to feature the real Thunderbird 2

There is another use of a hand in the same frame as a puppet (see 30 Minutes After Noon) as Captain Blacker is gunned down in the opening scene
For the first time, we see Thunderbird 4 launched from inside it's pod. In previous episodes, we saw it launched from outside the pod
Lady Penelope can be seen wearing the orange hat she wears in the title sequence when she is on board the Fireflash

This is John's only rescue, despite the fact he says he's been on a dozen or so. Scott comments he's been on them all. Unless he's forgetting he didn't play an active role in The Perils of Penelope, then this episode precedes both The Perils of Penelope and Atlantic Inferno

The Duchess of Royston is based on distinguished British actress Dame Edith Evans. Ray Barrett wonderfully provides her voice, also based on Evans
There is yet another use of a hand in the same shot as a puppet (see 30 Minutes After Noon and The Man From MI.5) as Parker pulls a gun on the crooked casino owner
The Domo is an adapted version of the Excavator (Martian Invasion)
The Empire State Building can be seen standing in the stock footage of New York. Either it was quickly rebuilt or this episode precedes Terror in New York City. Fans prefer to go for the former as real-life footage of it is shown as Skyship One flies over the city in Thunderbird 6

Baby crocodiles were used for the giant alligators. The RSPCA was anonymously called to the studio during filming for alleged cruelty to the alligators with electric prods. Upon arrival the RSPCA inspector, a Thunderbirds fan, said the animals could withstand a higher voltage and stayed to help.
Although he did not write this episode, Thunderbirds writer Dennis Spooner later reused the idea of an enlargement drug being washed down a sink, causing animals to enlarge for his script for the New Avengers episode Gnaws. In that, a giant rat was substituted for the giant alligator.
The stock footage of the thunderstorm was later used for the title sequence of The Prisoner.
Although she doesn't appear in this episode, some publicity shots were taken featuring Lady Penelope and the baby crocodiles. During the shoot, a crocodile got attracted to Penny's leg.

Maxie previously appeared as Bletcher in Martian Invasion. Pictures on Maxie's wall include one of himself (or Bletcher), Goldheimer (Martian Invasion), a Martian (Martian Invasion), the Duchess of Royston (The Duchess Assignment) and possibly Sir Arthur (Danger at Ocean Deep). Cass Carnaby also has a picture of the Duchess of Royston in his dressing room. The duchess herself can be seen at the Paradise Peaks Hotel when Penelope dances with Olsen

This episode features flashbacks from End of the Road, Sun Probe, Trapped in the Sky and Day of Disaster. These episodes were chosen as they had originally been filmed or scripted as 25-minute episodes
This is the last episode to feature David Holliday's voice of Virgil. Some sources say Holliday left because he had to return to America while others say he demanded more money

The inside of Tracy Island and the Thunderbird crafts have been completely redone throughout this season

The yellow construction vehicle previously appeared as the Houseman-Gray Vehicle in End of the Road but repainted

This episode features the largest guest cast seen in any Thunderbirds episode or feature film

The toys at the start of the episode are produced by J. Rosenthal (Toys). Unfortunately, Thunderbird 5 did not look like the genuine article, so it doesn't appear
New Thunderbird 3 footage is shown here: the launch was filmed for the feature film Thunderbirds Are Go
Jeff tells Nicky that the craft is 287 feet high
It is stated that Christmas Day will be a Sunday in 2067, when the episode is set; this is true