David Niven was partially dubbed by Rich Little as Niven was terminally ill with ALS (Lou Gehrig's disease) at the time and his voice was too frail to register.
This movie was intended to spin-off the character of Clifton Sleigh as the next Inspector Clouseau in a series of new Pink Panther movies, but it never took off. Eleven years later, a sequel was made, with Clouseau's son picking up the mantle in Son of the Pink Panther (1993), but this didn't take off either.
SPOLIER: Sir Roger Moore was a close neighbor of David Niven at the time of filming as both lived as tax exiles in Switzerland and had known each other for a number of years. Moore agreed to do this film incognito as a favor to help get the film made and to get Niven, who was in poor health, one last paycheck.
The story element of Inspector Clouseau turning to crime, with his new wife archcriminal, a.k.a. "The Frog", was taken from the second draft of the unfilmed screenplay of "Romance of the Pink Panther". Before Peter Sellers' untimely death, two drafts had been written for "Romance of the Pink Panther". When Sellers passed away, the project was cancelled.