Does anyone else make one joke, and then it becomes an ongoing joke? Like it doesn't matter what story you're writing, if you can reference it, you will? Or, maybe your stories make small references to each other or your real life. Or, maybe there's a concept all your stories share.
My favorite of mine is Jess Christy. My parents are religious and hated it when I would say "Jesus Christ", so I created my own female version. They have no idea that it's my replacement for the name. Now, certain characters in different books will say it instead of "oh my god" or "my lord".