Remember to keep strictly to the right hand lane unless you're passing! No loitering in left hand lane.
There are speed cameras in Germany. While the autobahn generally does not have speed limits, there are sections which have speed limits, e.g. near towns, construction, etc. and these often have low speed limits and speed cameras.
Insurance is complicated in a few countries like Italy and Ireland. I once rented a car in Italy and returned it in Switzerland. There was a small scratch on top of the rear bumper which I'm sure was there when I rented the car but the garage was really dark. Even though I had Hertz CDW, my regular car insurance, my credit card insurance, I still had to pay over 1000 euros.
Flashing headlights can be confusing: in some countries (France), it means "warning: stay out of my way"; in other countries (Germany, I believe), it means, "go ahead and pull into traffic".