When can you prepare a recently casted spell?

Question

The Pathfinder rules for divine spellcasting state that:

at the time of preparation any spells cast within the previous 8 hours count against the number of spells that can be prepared.

But does that mean that you lose a spell slot for that day, or can you use an extra session of preparation to prepare a spell in that slot, once enough time has passed?

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For example, my cleric got ambushed midnight and burned the 3rd-level domain slot. He can't prepare another 3-level domain spell on his preparation session on the morning, 4h after casting the spell.
But can he prepare a spell un that slot with an extra session once the full 8h have elapsed, or must he wait until the next morning and preparation session?

Answer 1

It means both, really. When you prepare spells, any spell slots you used in the past 8 hours cannot be used; you cannot prepare spells in them. Until you prepare spells again, they are empty.

However, you are allowed to prepare spells multiple times in a day, if you have spell slots open to put them in.

When preparing spells for the day, a divine spellcaster can leave some of her spell slots open. Later during that day, she can repeat the preparation process as often as she likes. During these extra sessions of preparation, she can fill these unused spell slots. She cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because she has cast a spell in the meantime. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if she prepares more than one-quarter of his spells.

So once it has been 8 hours since you cast spells from those slots, you can stop and prepare spells again to fill those slots (the ones you originally prepared remain fixed, however). This feature can be very useful even outside of this circumstance, since you can purposefully leave some spell slots open to prepare later, in case you need something unusual. Preparing spells later in the day requires 15 minutes.

Answer 2

You may only prepare it again during your daily meditation

As described under Spells of the Cleric class:

A cleric may prepare and cast any spell on the cleric spell list, provided that she can cast spells of that level, but she must choose which spells to prepare during her daily meditation.

This means that, other than leaving spell slots open to prepare spells later (taking an hour to do so), she must pick a specific time of the day (usually related to her god, but not necessarily), and she may only prepare spell slots during that period of time (or as soon as possible), as explained under Time of Day under Preparing Divine Spells:

A divine spellcaster chooses and prepares spells ahead of time, but unlike a wizard, does not require a period of rest to prepare spells. Instead, the character chooses a particular time of day to pray and receive spells. The time is usually associated with some daily event. If some event prevents a character from praying at the proper time, she must do so as soon as possible. If the character does not stop to pray for spells at the first opportunity, she must wait until the next day to prepare spells.

Spells cast during the 8 hours before the daily meditation count against the total slot spells available to prepare new spells, as explained under Recent Casting Limit/Rest Interruptions:

If a wizard has cast spells recently, the drain on his resources reduces his capacity to prepare new spells. When he prepares spells for the coming day, all the spells he has cast within the last 8 hours count against his daily limit.

As with arcane spells, at the time of preparation any spells cast within the previous 8 hours count against the number of spells that can be prepared.

While there are no developer comments confirming this, this is the interpretation that is mostly accepted by the community, as seen here, here, here, here, here, here, here and so on.

Finally, psychic spellcasters make no mention of such rule, and one would assume that they have no limitation, but it has been confirmed by Mark Seifter (Designer) that they also have a recent casting limit.

However, many people are fine with disregarding that rule, due to the complications it causes with spellcasters who prepare at a different time than right after a 8-hour rest (like clerics of Desna), or groups with characters who prepare their spells at different times, causing confusion at the table. As such, those people will (house rule) to completely refill their spell slots after a 8-hour rest, regardless if some spells were casting during the recent casting limit.