Ok. My GOD I can't believe this hasn't be caught yet, but... There is a tree that is both invisible, and can't be selected in ANY way by the edit too. In fact, the only way to remove these trees from your land is by returning ALL objects on your land.
I'm on my plot (which can only support 347 prims, and I'm going to be cutting that insanely close with the project I'm building) and just rezzing trees at random to see what happens. Occasionally, I hear the rezzing noise, but nothing happens, and I think nothing of it.
I go around collecting all the trees (after naming them... See, I'm trying to start my tree collection, finally, so I can custom-place trees if I want to), and I realize that I must not have all the trees, because I don't see them all. So I go back to rezzing. Slower this time. Again, occasionally I hear a noise, but nothing rezzes.
Thankfully, my entire build was set to (none) for a group, and I was rezzing these trees in the Wood Elf group, so the trees were one group, and my build was another. I set my land to Wood Elf, remove all visible trees, and... voila... there are three objects listed in the Objects tab that I can't account for. So I return them. They're all named Tree.
Then I stupidly turn my title off and rez one of these trees. Nothing appears. I try to bandbox select the area. Nothing. I look at how many prims are being used on my plot. It's gone up by one. I rez it again, and it's gone up by another one. I log out, and back in... Still no visible trees, but prims are being used.
Short version:
Invisible trees are using up prims for no apparent purpose on my plot, and I now have to return my entire build just to get rid of two (or five, not sure) prims.
Thank GOD just two weeks ago I had installed my handy-dandy build-rebuilder, just in case I ever had to do this. Just didn't expect it'd be getting a test so soon.
EDIT: I kan speel rael geeud.