You can leave a prepared spell slot open to spontaneously cast a spell.
Prerequisite: Must prepare arcane spells.
Benefit: If you leave an arcane spell slot open when preparing spells, you can use that open slot to cast any arcane spell you know of the same level or lower and of casting time no longer than 1 round. Casting the spell requires a full-round action.
You can use this feat only once per day, regardless of the number of slots you leave open.
Special: A wizard can select this feat as a wizard bonus feat.
Source: Complete Mage, page 37. Revised for errata.