Stones out of nowhere?

[Tolpatsch]

Hey there... sometimes I get stones floating, not connected to anywhere. Is it a bug, or is it supposed to be that way? I included a picture as reference.
In case it is not available, you can look at it via http://rcswww.urz.tu-dresden.de/~s4260857/Image.jpg

Sincerely yours,
Frank Muennich,
Dresden