David Hall: There are two major varieties for the 1950 Washington quarters, the D over S and the S over D. Both are quite rare with the D over S being somewhat more rare than the S over D. The story is that in 1950 the Philadelphia mint received an order for dies from one of the branch mints, and being one die short of filling the order, a mint employee overpunched the mintmark in an already existing die to create the die to fill the order. Then sometime later the exact opposite occured, creating the other variety.
Ron Guth: The 1950-D over S Quarter Dollar is one of the more popular varieties in the series. Mint State examples, while not common, appear on the market fairly frequently in grades up to MS65. In MS66 and better, this variety is very rare; the finest example certified by PCGS as of 2/9/2011 is a single MS67.
On this variety, the mintmark looks like a D over D over S, with the top and left of the S appearing underneath the upper left side of the final D. This variety is also known as FS-601 (old FS-021).
Sources and/or recommended reading:
"Cherrypickers' Guide to Rare Die Varieties of United States Coins, 4th Edition, Volume II" by Bill Fivaz and J.T. Stanton