I disconnected my battery recently, and when I refitted it, the odometer mileage reading reset itself to zero - not the trip reading, but the total mileage for the bike.
When I took the bike to my Yamaha dealer for it's annual mot test last week, he said there are quite a few batches of Yamaha speedo's that do this, and he thought they all came from the Italian Yamaha factory.
This is a 1993 fairly tatty bike used solely for green laning. I bought it cheap, and would not expect much residual value when I sell it, so this doesn't bother me at all. On the bright side I may well be able to blag a free 500 mile service soon!
