I recently rented a car in Germany (I'm from the US) and on one part of the trip between Müncheberg and Berlin, I don't think there were any speed limit signs posted. Once I realized this, I had to rely on the signs signifying the entering and exiting of the commercial areas.

Today, I received a the notice from my rental car company with the speed camera printout. Sure enough, it was from the area where I first realized there were no limits posted. Do I have a valid argument? I'm sure people are expected to know when they enter a reduced speed zone, but this was literally the first time after driving 1500 km where the limits weren't posted.