I apologize if this has been posted before, but the Lindens I talked to didn't seem to have an answer, and since I figured it out, I wanted to share.
I was tearing apart my sim as part of a rebuild. In the old build, I had built a fake "ground level" approx 40 meters over the real "land" of the island.
On a whim, I had whipped out some Linden grass (the 1-prim freebie grasses that doesn't look all that bad from the Library) and put it on top of my fake "ground". I didn't see it rez, so I did a few more.
Eventually I realized when I was down around 0m that the grass DID rez, it had just snapped to be level with the "real" ground, 40m below where I had rezzed it.
I learned that this is how Linden grass is meant to behave - it is programmed to sit directly on the "real" land, not necessarily where you rez it.
Long story short, time came that I wanted to delete this Linden grass, but couldn't.
Every time I got close, either with camera or avatar, the grass would disappear. If I backed up or zoomed out, it came back. I could not select the grass. I could not drag-select it either.
The Lindens in live chat said that its a known bug and sometimes Linden grass will randomly attach itself to other prims and you can't delete it. I was told that even the Lindens could not grab it and delete it for me. So I was seemingly stuck with this phantom grass that was there, but not there.
For my new build, I had everything cleared off from the ground, and wanted to use "real" ground this time around, and proceeded to raise the land up from 0m, watching the unwanted grass happily move with it.
On a whim, I kept raising the land until it was at 40m - the height where I had originally rezzed it from to begin with. Lo' and behold, I could select it. It didn't disappear when I got close. Most importantly, I could delete it!
Moral of the story: If you rez Linden grass from an altitude above your "real" land on the island, be prepared to live with that grass forever, unless you can raise your land up high enough for the SL engine to let you grab it.