I've had it happen on various occasions on different bikes, and it's always a sloppy shift to blame..
Usually about once a year I catch myself with a bad shift and the bike lets me know what I did.. This year on the Wing, I was downshifting and I didn't do my job and the bike slipped into a false neutral..
If it starts happening on a regular basis, it's a sign of a worn gear or bent shift shaft or fork.. If it's just every great once in a while, it's a sign of the rider thinking about the ride..