If I dropped a washer into the head, and couldn't find it I would tell myself off sternly, then carry on with life.
The Washer Will Find A Safe Place to live in the sump, hiding in the corner away from the spinning bits.
Then every oil change I would look through the oil with the excitement of a child at Christmas hoping to find the washer, but only to be disappointed time and time again.
I am more worried about the swapped caps rather than the washer. Unless by cap you mean splash guard.