strange graph behavior - random machines & graphs

Some additional information...  If I delete the RRD files that become
corrupted like this and let them rebuild, not only does it rebuild it
corrupted again, but if you click on the graph for the full history, all
four of the historical graphs show the exact same corrupted data.  That is,
it has a full 576 days worth of corrupted, invalid data, even though the RRD
file has only had data going to it for a handful of Hobbit polling cycles.