Some of the free actions (Force Workers or some super projects for example) require you to "lose a worker." I cannot find any clarification in my searching as to whether the lost worker needs to be active (or can be tired). Also, it's not entirely clear whether you are permitted to lose a worker that has already been placed during that Era (thus potentially opening up an action spot for a second use).
My suspicion is that it's permitted to lose any worker that you have not already placed (whether tired or active) but probably not permitted to lose a worker that has been placed that Era. I would love some clarification of this matter.
EDIT: It seems my guess was correct based on the "Frequently Overlooked Rules" section.
1. You can never use a busy e.g. placed worker, to pay a cost.
2. you can use a tired or awake worker to pay a cost.
3. Some actions mean you lose/kill the worker , when you retrieve them, they go back to the supply. These normally have a skull on the action space e.g. anamolies and some buildings/superprojects.
4. Returning a worker to reclaim a warp tile must come from the awake reserve.
This is all in the rules.