One of my headlights was rattling in its mount and because the castle-nut had already bottomed out, I figured on slipping a washer in there to take up the extra room. That didn't work because a standard washer was too thick to allow the castellation to center on the cotter-pin hole.
I was figuring I'd file down the nut to the point where things would line up and while reaching into the tool box I'd inherited for one of my Dad's old machinist's files, I saw a little jar of star-washers. Haven't seen or thought about them in years.
I remembered something about how they're supposed to be a one-use-only replacement for a lock-washer, but they're quite thin when you crush them down with a moderate tug on the wrench—exactly the right thickness to shift things just a little to accommodate a cotter-pin.
They're also great for ensuring an electrical ground because the teeth dig through the paint into the metal underneath.
Technically, a split lockwasher is also a one-time-use device.
I am at the point where I just replace most split washers with new ones.
It saves cleaning the old ones, reduces the chances of the old one breaking, saves time and looks better.
On electrical parts it can even better insure a good ground.
I now buy 1/4, 5/16 and 3/8 washers by the box.