* In the earliest books, Chet Morton was often portrayed as an irrepressible prankster, and often the books would have a chapter concerning one of his pranks as filler material or to help the plot (IE: in "Tower Treasure", to prevent the bumbling police detective Oscar Smuff from ruining Fenton Hardy's interrogation of a dying criminal, [[FunnyAneurysmMoment Chet comes up with an idea to make a fake "bomb" by putting an alarm clock in a box and hiding it under a fruit stand, and has another boy find it under there. The proprietor alerts Chief Collig and Detective Smuff, causing a major fuss and thus making the policemen miss their train]]). Sometimes it could come off as being a bit douchey.
* After being set up with fake credit cards planted in their van and interrogated by Chief Collig, he dismisses them easily, but in a gruff manner. And then Joe asks:
-->''Are we still friends?''
* The always-stoic Gray Man once treated the boys to a meal, and he complains that they almost ran his paycheck dry with the bill.
* Bowling as ''Aunt Gertrude's'' hobby is funny in of itself already, but the fact that she does it to ''relieve stress'' makes it even more hilarious.
* At the end of ''Acting Up'', a Bollywood star they befriended decided to have a little fun and directed a few of his fans to ask Frank for an autograph, introducing the latter as a foreign star. Joe couldn't keep a straight face and wanted to kid Frank about it, but Frank got the last laugh when he told Joe he signed the autographs in ''Joe's'' name.
* During an undercover mission infiltrating a foreign military base, Frank gave what he ''thought'' was "Here are our passes." to a guard in the latter's native tongue, but the guard immediately raised an alarm, and the boys' cover was blown. At the end of the story, Joe consulted their translator, and told his brother what he really said. "Here are our ''shoes''."
* In the original series' "Secret of the Lost Tunnel", the heroes have to solve a hundred-year-old puzzle to help them find a gold cache that has gone missing during the American Civil War. Chet ''thinks'' he's solved the puzzle, and offers this analysis:
-->'''Chet:''' "The CSA stands for 'Can't Stand the Army.' The guns stacked up means they're going to stop fighting and sit down under that tree and eat breakfast. That big round thing's an egg."
-->'''Joe:''' "What about the numbers?"
-->'''Chet:''' "That's just to confuse us!"