Well, encroachers sometimes simply act in error and the matter is easily dealt with. When folks build in the air they can lose track of propety boundries really easy. Or sometimes SL belches and things suddenly find themselves where no one expected them to be, like one time I came home to discover a dining room set in my attic! The neighbor was putting it in and it disappeared from her house and ended up in mine.
But then there are the occasional unfortunates who for some reason don't quite understand that we pay good tier for our land and we are somewhat jealous of our air rights. Recently someone decided to use a sandbox next door to me to put up his house and as his bad luck would have it, his bed just happened to be on my land. So as I stood on the roof (on my property mind you) and cammed in, the opportunity for some legal nastiness was just too good to ignore, so I dropped a nice big prim right into the middle of his bed.
The purple cube was a wonderful idea. I will have to build one and keep it in the inventory for just such an occasion.