What credit card mistake did you make (and learn from)? Let your miles expire? Late payment?

For an upcoming story on a financial website, I’m looking for stories about people who made a credit card mistake and learned from it. Did you make a late payment and lose your introductory rate? Let your miles expire without using them? Rack up debt on a card with a too-high interest rate? How did you change your behavior after? Please send your story along with your full name, age, and city.