About 20 years ago I was on an prop commuter from BWI to PHL. On climb out the pilot announced flight to JFK. I thought it was a mistake but the flight path was pretty far offshore for a PHL flight. I thought I got on the wrong flight so I asked the guy across the aisle and he said PHL also. No flight attendant so leaned forward in my seat and started tapping on the curtain and eventually the copilot pushed the curtain aside and pushed his headset off one ear. I said his announcement said said JFK but we were all going to PHL. He just nodded and pulled the curtain back. About ten minutes later we bank left and change course about 40 degrees.

I asked a friend who worked for the airline when we landed and he said the pilots got on the wrong plane and picked up the clearance to JFK for their originally scheduled flight number.