Following my previous post, my viewer has officially declared war on me, by first (apparently) duplicating the entire contents of my landmarks folder, and then, once I had painstakingly de-duplicated, informing me, "Aha! I didn't really duplicate them after all! You have just erased all your landmarks, ho ho ho!"
Maybe the viewer expects me to crumble under this devastating onslaught, but it's not actually as big a blow as you might think, given that all my landmarks are 1) places I go regularly, so easily recoverable from my teleport history, or 2) places where I buy stuff, so I can get LMs from my shopping folders, or 3) places that can easily enough be found in Search, so I can recover them that way, or 4) places that actually don't exist any more, so really I should have purged them anyway.
Besides, I have been in SL nearly two years now, I was due a good inventory-related catastrophe. And, really, deleting 500+ landmarks has resulted in a 1% slimming of my bloated inventory, so it's all to the good when you look at it.
So... not a knockout blow, viewer. Try harder next time.