When using sand, desert sand, or gravel to fill in and drain liquids, and re-mining said node, I often find I have less than nodes than when I started. In addition, whenever someone on a server I'm playing on has been working with sand, desert sand, or gravel, some of those nodes will start popping up where other people are building, mining, or whatever, sometimes even replacing other nodes. From what we can piece together, we think there is something wrong with falling nodes, and that when they fall, they are sometimes placed at drastically incorrect coordinates. Because they only seem to show up where people are working, I think that if the falling node is incorrectly placed in a section of the map that isn't loaded, it just disappears forever, but if it lands in a section that is in use, correctly or incorrectly, it goes back to being a regular node.
I don't know if this has been reported yet, but I didn't see a topic about it. It seemed to have started with the release or 0.4.4 though. I've just been to lazy to post about it.