It seems like German traffic engineers sometimes like to be sadistic. One way they do that is with the arrows on parking-related signs. To wit, you will occasionally come across parking signs posted like the one in the photo below:

Wait, aren't the cars at the left parked illegally?

Since the arrows on the sign point to both sides of the street, it would seem that the cars on the left side of the street are parked illegally, right?

This is what it seems like the sign is indicating.

However, although the sign is faced perpendicular to the roadway, parking restrictions always apply only to the same side where the sign is posted. Therefore, the arrows actually mean in both directions from the sign on the same side of the street. You may have to turn the sign in your head so that it faces the street for it to make sense, especially for signs with a single arrow on them.

You have to mentally rotate the sign like this to envision what it really is indicating!

So why do they post the sign perpendicular (or nearly so) to the roadway in the first place? It's actually not to confound drivers. Rather, they do that so it will be visible to oncoming traffic on that side of the street, like they do with most signs. But, to alleviate the confusion for foreigners and Germans alike, parking signs are more often angled like the ones shown in the photo below. This helps to maintain good visibility while improving the understanding of which way the arrows are pointing.

(Photo from under38.de)

This same logic applies to all parking-related signs with arrows: