Years and years ago, I read a book on cryptography that I picked up because it looked interesting--and it was!
But there was a side anecdote in there that stayed with me for more general purposes.
The author was describing a cryptography class that they had taken back in college where the professor was demonstrating the process of "reversibility", which is a principle that most codes depend on. Specifically, it should be easy to encode, and very hard to decode without the key--it is hard to reverse the process.
So he had an example code that he used for his class to demonstrate this, a variation on the Book Code, where the encoded text would be a series of phone numbers.
The key to the code was that phone books are sorted alphabetically, so you could encode the text easily--picking phone numbers from the appropriate alphabetical sections to use ahead of time would be easy. But since phone books were sorted alphabetically, not numerically, it would be nearly impossible to reverse the code without exhaustively searching the phone book for each string of numbers and seeing what name it was tied to.
Nowadays, defeating this would be child's play, given computerized databases, but back in the 80s and 90s, this would have been a good code... at least, until one of the students raised their hand and asked, "Why not just call the phone numbers and ask who lives there?"
The professor apparently was dumbfounded.
He had never considered that question. As a result, his cipher, which seemed to be nearly unbreakable to him, had such an obvious flaw, because he was the sort of person who could never coldcall someone to ask that sort of thing!
In the crypto book, the author went on to use this story as an example of why security systems should not be tested by the designer (because of course the security system is ready for everything they thought of, by definition), but for me, as a writer, it stuck with me for a different reason.
It's worth talking out your story plot with other people just to see if there's a "Why not just call the phone numbers?" obvious plot hole that you've missed, because of your singular perspective as a person. Especially if you're writing the sort of plot where you have people trying to outsmart each other.
