First Walt Disney Productions movie appearance by Dean Jones (F.B.I. Agent Zeke Kelso), who went on to star in numerous Disney projects throughout the 1970s.
The Seal-Point Siamese cats who collectively played the role of D.C. in this movie were all "traditional" Siamese, as opposed to their more streamlined contemporaries. Amongst fanciers of traditional Siamese, they're affectionately known as "Appleheads".
The 1964 Mercedes 230 SL that appeared in the film as Roddy McDowall's car was actually Walt Disney's personal car. Disney rented his car to the film for $100 per day, possibly because he felt guilty about buying himself such an expensive car. The Disney family still owns the car to this day.
When Patti Randall meets Agent Zeke Kelso for the first time in his F.B.I. office, she says that her father named the cat. She says her father called the cat "something much worse" than "darn", but her mother wouldn't let them use that name. The inference is that Patti's father used the word "damn", which was the case in Gordon Gordon's and Mildred Gordon's source novel, "Undercover Cat."
In the original 1963 novel, "Undercover Cat", "D.C." stands for "Damn Cat". Even if the motion picture code was beginning to loosen up at the time, such language would not have been welcome in a Disney movie.