If your car is equipped with Multimedia Interface 2G, then it's good to know what's the force reboot key combination. If you navigation screen freezes you can restart the system without turning the car off (e.g. navigation app crashes while driving).
Press at the same time three buttons:
- SETUP (in A4, A5, A6, Q7) or PHONE/TEL (in A8)
- upper right soft button
When you release those buttons, MMI 2G screen will go black and after few seconds boot logo will appear.