There are two major error varieties of the 1950 Washington Quarter. A 1950 D over S mint mark and the 1950 S/D both are quite rare.  The 1950 D/S is the rarest of the two.  It is very interesting how errors like this can happen at both the San Francisco and Denver mints and in the same year. This is my favorite mint error as it makes me wonder how an error like this could possibly happen. 
This coin is a must-have for any Washington Quarter collect.  Great valve for this rare mint error.  Very pronounced error..


Very pronounced D over S, see the photo.