If you build an eng in a Sera airfab and the unit is finished, it takes a few seconds before the unit is "ready" to be selectable while it is getting unloaded from the fab.
In the seconds you need to wait, you already could have queued some orders. For example: Cybran eng can be instantly selected while it is unloading from its airfab.
This game mechanic should be common for all races. The waiting time adds up over time and is just annoying. Let me select the eng when it is finished to give it some orders, so I can be busy somewhere else and forget about the eng.