Frequently missing (and easy lose) but essential part for the functionality of the door check strap.  

One washer is required for each pin on each door (4 washers per sedan/wagon, 2 washers for coupes).

If yours is missing, your door check strap will be prone to getting out of alignment and wear out prematurely. These plastic washers serve an important function by ensure keeping the alignment of your door check strap proper and square.