The neighbor lady toss rocks on your lawn from working in her flower bed. Neighbor kids leave toys, tennis and golf balls and... claw hammers!!! (forgotten in the grass after building their secret day fort in your back yard). Wine bottles from the other neighbors garden party. A hiking boot from... I have no idea!!! And don't forget stuck sprinkler heads. Spray paint cans from the local juvenile delinquents... On and on...
The workaround, which I didn't implement yet, is to build a smooth and clean garden. Of course branches fall, ants build, all sort of animals dig, bushes grow id you give them a couple of weeks at the wrong time of the year, etc.
That lawnmower is also much bigger than mine.