It is when you find yourself driving IRL and race sim habits come into play!

For example:
1) After changing gear in ACC with paddles you attempt to do the same using the indicators and light stalks in your road car!
2) You set stage times for a road trip much to the annoyance of your passenger.
3) Refueling stops do not allow time for 'comfort breaks' (much to the extreme annoyance of your passenger)

Feel free to add more from personal experience......