Author Existence Failure: Peter Sellers died during planning of the unmade Romance of the Pink Panther. Initially there were plans to film it anyway as a Dudley Moore vehicle, but he wanted Blake Edwards to direct it, and he didn't like that script; Moore moved on while Edwards made Trail and Curse instead.
Cash Cow Franchise: Even more so when you add in the animated spinoffs and their associated merchandise.
Fake Nationality: Peter Sellers was an Englishman playing a Frenchman, but beyond this, the character of Clouseau affects several disguises in the course of the series that require him to be various other highly-caricatured nationalities.
Herbert Lom (Commissioner Dreyfus) and Graham Stark (several roles) are also Eastern-European and British respectively. Sellers and Lom already co-acted together in "The Ladykillers"
Italian Maria Gambrelli in "A Shot in the Dark" was played by German Elke Sommer.
Fake Russian: Lesley Anne-Down (British) as the Russian spy Olga in Strikes Again.
Franchise Killer: Oh, boy. Where to even begin? This franchise was killed at least three times in its history. The original series was killed by the one-two punch of Trail of the Pink Panther and Curse of the Pink Panther (both of which were made without Peter Sellers' involvement; he had died by that point), both of which were box office failures and lambasted by critics. A decade later, an attempt to resurrect the original series with Son of the Pink Panther ended up becoming an enormous failure and sealed the original series' fate for good. The Continuity Reboot series with Steve Martin had an even shorter life. While the first film did well at the box office despite negative critical reception, the sequel three years later not only received more negative critical reaction, but it ended up under-performing the first film and put the series back on the back-burner once again. Time will tell whether or not the series gets rebooted yet again.
I Am Not Spock: Averted with Peter Sellers, David Niven, and Robert Wagner; played straight with most of the others.
Missing Episode: Sort of - MGM/UA doesn't own the home video rights to Return (currently, Universal/Focus Features does) so while all are available on DVD, there's never been a proper box set of the five "true" Sellers films. MGM is also not allowed to mention Return in any published material. They do however hold the rights to release it digitally.
Money, Dear Boy: Partial motivation for Sellers being willing to reprise the role of Clouseau in the 1970s. (The other was getting enough clout to get Being Theregreenlit.)
Old Shame: Alan Arkin, not surprisingly, considers making Inspector Clouseau a mistake. He has rarely spoken much about the film in the years since, but one time Arkin did explain why he took up the role. In the late 1960s he was becoming a fairly well-known actor and thought that as a movie actor, he could do whatever role was offered to him, and he could do it easily. He admits that the failure of Inspector Clouseauknocked him off his high horse.
The Other Darrin: The Clouseaus besides Sellers, of course, but several other recurring characters were played by different actors from film to film (see the trope entry) in the classic series. In the rebooted series, Kevin Kline played Dreyfus in the first entry, but was replaced with John Cleese in the second.
Serendipity Writes the Plot: The animated title sequences usually had a theme relating to the film's plot, however many fans have noted that the opening sequence for Strikes Again feels like it's own short film, and doesn't connect with the actual movie's story (it involves Clouseau trying to catch the panther in a movie theater, and the two being transported into the screen). The director of the sequence, Tony White, explained once that, for unknown reasons, Blake Edwards never sent Richard Williams Studio a copy of the script to draw inspiration from, thus they had to create their own ideas independently.
Star-Making Role: Peter Sellers was already a huge star in the United Kingdom thanks to The Goon Show and a list of TV and film credits that was already in the double digits, and had made inroads in North America with The Mouse That Roared and his work with Stanley Kubrick, but Inspector Clouseau was the role that made him an international success.
Technology Marches On: In Strikes Again The insane man who breaks into the television broadcast is identified through a photograph someone took off a TV screen.
Throw It In: Occasionally; for instance, the "synchronized watches" bit in Shot was largely improvised by Peter Sellers and Graham Stark.
Also Clouseau's line 'a rit of fealous jage' was supposedly a slip-up (there is an outtake where Sellers and George Sanders both crack up at it) which was later re-filmed as part of the script.
Peter Ustinov was originally cast as Clouseau, but left over a salary dispute.
Sophia Loren and Walter Matthau were considered for Shot (Matthau had appeared in the stage play the film became an In-Name-Only adaptation of) but were replaced when Blake Edwards stepped in.
The Revenge shoot in Hong Kong was almost derailed by monsoon season; the threat of the rain keeping up had Blake Edwards having his assistants check up on Rio de Janeiro as an alternative location.
The Le Club Phut sequence in Revenge originally had Clouseau and Cato masquerading as black clubgoers, funny Afros and all. However, Peter Sellers and Blake Edwards's increasingly strained working relationship got in the way and they warred over how they would pull it off; ultimately the sequence had to be scrapped and substituted with the simpler sequence of the two trying to spy on the club from outside.
Blake Edwards wanted to cast Rowan Atkinson as Clifton Sleigh in Curse, but United Artists (an American studio) didn't think he was enough of a "name". He later considered casting Atkinson as Clouseau, Jr. in Son; other potential leads included Tim Curry, Gérard Depardieu, and Kevin Kline (who eventually played Dreyfus in the 2006 reboot).
With regards to the 2006 reboot, potential Clouseaus included Kevin Spacey, Chris Tucker, and Mike Myers. Myers was the most sought-after, but his asking fee was too high.
In the early stages, Paul Giamatti was considered for the role of Dreyfus in the reboot, and there were even plans for a Running Gag that would have had the Pink Panther animated character worked into the actual story by having him appear as Dreyfus's madness-induced hallucinations.
Plans to incorporate Cato into the reboot were scrapped when the filmmakers couldn't get Jackie Chan for the role.
The reboot originally had a stylized title sequence where the Panther was animated with CGI. It was dropped and was replaced with a traditionally animated opening with the Panther and Clouseau.
Early press releases for the reboot stated Steve Martin was going to play an older version of Clouseau's son. It's unknown if these were just misinformed sources, or if that was in fact the original plan.
Composers: For the theatrical shorts, Bill Lava composed for every short from 1964 to 1966. He was succeeded by Walter Greene, who only scored a few cartoons before his cues (plus some by Lava) were recycled for every short well into the mid 70s. By 1978, an uncredited Steve DePatie started composing new cues that were used until the series concluded in 1980.
Keep Circulating the Tapes: Available only via dodgy torrent sites in Eastern Europe, and Kamuz TV's Argentinian-Spanish copies, but no official DVD release of all three series exists (for this 1993 one). You could get a DVD at one point, but it was only of Series 1, the 1993 series.
So it's left to fan petitions, then... for the full series release.
The complete first season is available on Amazon.de, if nowhere else.
The Merch: In an odd example, the Pink Panther character is also the mascot of Owens Corning insulation, leading to weird moments when kids discover the product in home improvement stores.