For example, say a node with member id 2 in the config crashes. The data is restored from backup taken a day prior and restarted. Now the node begins reporting upstream for id 2 a position that is behind where id 2 had previously reported.
Today's position managing code does not change a node's position if it is reported to have moved backward.
This is a problem because nodes can thus appear to contain data that they do not actually have.