During our last stay in Berlin we were lodged at a hostel in Fasanenstrasse, close to Kurfürstendamm (Charlottenburg). The pension was in an old building which was protected by some urbanism laws which prevented the owners to perform some works.
Thus, the building maintained a curious key lock system at the front door which was activated by night. The key was double sided as it can be seen in the picture below.
In order to open the door you would have to introduce the key in the lock, unlock it and then fully insert the key in the door until it would appear in the interior of the house. You would then have to lock it from the inside and remove the key from the inside by pulling from the other end of the key.
We found it funny and curious, so we recorded this short video showing the system: