There seems be a (very annoying) bug, not sure if it's mc, bukkit or addon related. It doesn't happen every time either, but when you break an item off a block, sometimes when you come back later the block will be missing. I had an op check one of these spots, and hawk reports that I broke it myself. I certainly did not break it myself though, and this has happened 4 or 5 times to me now, and at least once to someone else. The first time it happened to me when I was harvesting potatoes, one of the tilled dirt disappeared under me. The other bunch of times, it's been a torch that I placed and broke off, and later the block was missing.