When Don Adams was negotiating his salary, he had his choice between more money per week and no ownership stake in the show, or less money per week and part ownership. Adams chose the ownership deal and never regretted it considering the series' durable popularity in syndication gave him a regular income even as he struggled with being typecast by it.
CONTROL and KAOS were supposed to be acronyms, but Mel Brooks and Buck Henry never came up with anything for them to stand for.
When asked how he developed his trademark voice for Agent 86, Don Adams said that it came from watching "The Thin Man" movies starring William Powell and Myrna Loy. After hearing Powell repeatedly say, "Asta! Asta!" he decided to imitate his voice for the character of Maxwell Smart.
Mel Brooks felt terrible when he saw the opening titles for the first time and saw that they read "created by Mel Brooks WITH Buck Henry" because it seemed to minimize Henry's contribution. Brooks was not involved with the series beyond the pilot, whereas Henry stayed on as script editor.
It is rumored that Agent 99 was originally supposed to be named Agent 69, but NBC censors deemed it to be too "sexually suggestive". According to Barbara Feldon, this is not true. Her character was originally to be named Agent 100 "because she was one hundred percent", but Buck Henry decided 99 sounded more feminine.