When a pregnant woman refers to her pregnancy progress in weeks, not months.
Person A: "Wow, you're pregnant! How far along are you?"
Pregnant chick: "I'm 20 weeks."
Person A: "Oh so 5 months?"
Pregnant chick: "No, 20 weeks. It's the maternity metric system equivalent for 5 months. But saying it in weeks makes me sound better than everyone and it confuses and annoys people."