A few oddities of OpenStreetMap history files

I've been working with the OSM history data for Great Britain for a while now. This has mainly removing bugs in the initial processing (yup, out-by-one errors mainly) and, as a side effect, working out how to improve the speed of extracting way geometries. En route I have noted a few quirks which may be helpful for others working with the data.

Some of these should be obvious, but I feel that they are worth stating nonetheless.