This overdate error coin is a very popular variety which resulted when a die blank was first impressed with a hub dated 1917 and then received a second impression from one dated 1918. Such errors were possible late in the calendar year, when dies for use in either 1917 and 1918 were being prepared. By sheer chance, this die was then stamped with an S mintmark for shipment to San Francisco.
 
Whether or not anyone at the Philadelphia Mint's Engraving Department or San Francisco's Coining Department noticed the overdate is uncertain. The demand for fresh coin was so great during the war years that this die may have been used even if detected. This variety was not reported until the early 1930s, by which time there were very few surviving rolls of 1918-S quarters to search.
 
The coins that entered circulation quickly lost their dates, as was typical of this coin type, so it's doubtful that more than several hundred examples are known today in all grades combined.
 
No wonder avid error coin seekers will want to scoop up this very scarce error coin, no matter the cost. Don't wait too long because the price is about to go up!