at cross-purposes

Meaning: If you're at cross-purposes with someone, you think you're both talking about the same thing but you're actually talking about different things.

For example:

• I was talking to Kyle about Georgia, but we were at cross-purposes. I was talking about Georgia in Russia, and he was talking about Georgia in the U.S.A. We laughed when we realised we were talking about two different "Georgias".
  
• Jill and I were at cross-purposes today when we were talking about John. I was talking about John Smith, my neighbour, but she thought I was talking John at work!