sregidor@redhat.com brought up this issue, so all credit to him (: 

Stubs secrets(like worker-user-data) are often shared between MachineSet objects. When https://issues.redhat.com/browse/MCO-1485 lands, stub secrets referenced by MachineSets are automatically upgraded to spec 3. This is not a problem for a cluster that is fully opted in for boot image updates, but the MCO also allows a partial selection mode, where only some MachineSets can be selected for boot image updates. In such a partial selection case, the unselected MachineSets could potentially have an old boot image that is unable to use a spec 3 ignition stub. This would break scaling on the unselected MachineSets. We should specify this in the documentation.

Key points:

• This limitation is only for clusters whose stub Ignition secrets(*-user-data) are in the spec 2 format and plan to use the partial selection mode for boot image updates.
• Prior to opting in a cluster to using partial selection mode, the cluster admin should backup their spec 2 stub secrets.
• If they wish to scale their unselected MachineSets, ensure that those MachineSets reference the backed-up stub secrets in spec 2 format.
• Reiterate that partial selection mode is mainly meant for testing purposes and we expect most cluster admins to opt-in all MachineSets for boot image updates