PCGS 1979 Washington quarter struck on a nickel planchet. The wrong planchet coin is slightly smaller than a normal quarter, and is struck on the planchet of a Jefferson 5c.

Washington quarters struck on nickel planchets are the most affordable type of quarter wrong planchet error, and a collection by date would be attainable with some searching, since examples are likely available for every date of post 1964 quarter on nickel planchet. 

The error occurred when a nickel planchet was fed into a press that was striking Washington quarters and was struck, resulting in a coin with the design of a quarter but on the planchet of a nickel. This mixup can occur when a bin used to transport loads of planchets to the coining presses is used for nickels and then is not completely emptied--perhaps a few become lodged in a corner of the bin. If the bin is then used to transport quarter planchets, the few nickel planchets may become dislodged and fed into the quarter press along with the quarter planchets. The result will be a wrong planchet error quarter on nickel planchet!

MS65