and don't forget the cotter pin.
I had the scoot lying against the mulch pile last night to check for a hub seal leak; the rear brake had lost its friction and I was sliding for a mile.
Turns out the hub nut (which I had torqued up properly last year) had loosened to finger-tight. The weird nut cap thing and the pin prevented disaster.
A can of brake parts cleaner and a new hub seal (and a proper torquing) put me back on the road.