Internally pedigree stores the selected proband as a link to an internal pedigree node (internal ID). When a node is deleted internal IDs may change, but proband link is not updated on delete.
(technical details: internally pedigree nodes are always renumbered so that there are no gaps (for performance reasons), so e.g. if there were nodes [0,1,2,3,4] and node 2 is deleted nodes 0 and 1 will stay as is, but node 3 will become new node 2, and node 4 will become new node 3, and there will be no node 4 any more)
So there are actually two bugs:
1) proband node may suddenly change (if proband node id was higher than the deleted node id)
2) if proband node had the highest ID (e.g. the node created last), and any other node is deleted, the internal proband pointer will now point to a non-existent node which may crash pedigree editor in some cases. This is the bug ran into