Again, consider a sloped lawn. Normally, if
the mower encounters the perimeter, it will
stop, backup and rotate such that it faces away
from the perimeter. However, when going down
hill, it may generate enough momentum to actually
carry it beyond the perimeter.
Oh and there is one other much less common
scenario. Rain. The mower is smart enough to
go back to its charging garage if it is raining,
but unless Iím missing something, it will simply
sit in the garage until someone pushes the appropriate
buttons to make it try again.