I was experimenting in Potlatch last night with trying to add an outline of Castletown, Isle of Man and all seemed to be going well. Until, that is, I downloaded that same area in JOSM so I could use some WMS mapping data for reference.
Pretty much everything that was completely enclosed within the new outline I'd just drawn in Potlatch didn't seem to exist in the data JOSM was showing me. Most of the streets (nodes, segments and ways) of Castletown had disappeared from the dataset it seemed.
Confused, I checked back to a previously saved copy of the data and got some IDs to check against the history data through the API. The ones I checked were all showing as being there. And sure enough, when I started JOSM from scratch, the data was once again there.
It was very weird, and I still don't understand why it happened... It sounds like a bug somewhere, but where, I don't know!