Does anyone remember the story about the famous mathematician at (I think) Princeton, who would always pick 3 as a random number in his examples. ("Let's pick a random number, say 3...")

And then he ended up mentoring an entire generation of mathematicians who adopted his habit.

And everyone would always "randomly" pick 3. It became a thing.

It's kind of a variant on the "let L be a group" because the joke relies entirely on the social conventions of a small group of people.