From: Mike Pall <mike>

This is a serious bug. But extremely hard to reproduce, so it went
undetected for 8 years. One needs two resurrections with different
main nodes, which are both in a hash chain which gets relinked on
key insertion where the colliding node is in a non-main position. Phew.

Thanks to lbeiming.

backport 046129dbdda5261c1b17469a2895a113d14c070a