John-I noticed that my case drain on my kTm was leaking ever so slightly, so I tightened the plug a bit, but realized that it was pretty tight to begin with. There was that "Crack!" noise from the flange on the bolt head moving against the case. Potentially a catastrophic situation here, with the aluminum threads and all. I got lucky. Next oil change, I noticed something fall into the funnel as I was emptying the drain pan into the waste oil jug. It was the copper washer that had fallen off the drain bolt and into the drain pan from two oil changes ago! Sometimes ya just get lucky!
The only issue I can see is to make sure you get a copper washer and not one of those "Crush" washers, as they will stay flat after a use or two.