Insufficient resources to satisfy vsphere HA failover

I was in the process of taking one of two ESX cluster nodes offline (into maintenance mode) but ended up with following error message “Insufficient resources to satisfy vsphere HA failover“. After much troubleshooting, it turns out that when the vSphere HA Admission Control Policy is enabled, a percentage of the ESX resources is reserved for HA use in the event of a failover as it has to be able to handle all possibilities.