The Uncanny Forethought feat (Exemplars of Evil, p. 26), among other benefits, allows a spellcaster such as a Wizard to leave a spell slot open and then later:
[...], as a full-round action, you can use a reserved slot to cast any spell that you know. The spell is resolved as normal, but for the purpose of the spell, your caster level is reduced by two. The level of the slot used must be equal to or greater than the level of the spell you intend to cast.
However, is not clear what a known spell is for a Wizard; does this mean:
- A spell known through Spell Mastery?
- A spell known, as in written in one's spellbook?
- A spell known, as in written once in a spellbook? (which may have since been lost)
- A spell known, as in on the Wizard spell list?
- A spell known, as in on the character's spell list? (which may have been extended)
- Something else entirely?
Looking through the SRD, the only mention of known spell in relation to a Wizard is:
Unlike a bard or sorcerer, a wizard may know any number of spells. She must choose and prepare her spells ahead of time by getting a good night’s sleep and spending 1 hour studying her spellbook. While studying, the wizard decides which spells to prepare.
Which does not define what know means, at all.
I am not aware of any errata, and searches proved fruitless.
I would note that (1) is pretty unlikely to be the meaning, here, since spells known through Spell Mastery have a dedicated entry in Uncanny Forethought.