Casting nerveskitter consumes the caster's next turn's swift action
As is typical for an immediate action, a caster that takes an immediate action to cast the 1st-level Sor/Wiz spell nerveskitter [trans] (SpC 146–7) uses his next turn's swift action to do so.
However, because the description of the immediate action says, "You… cannot use an immediate action if you are currently flat-footed," it took the Spell Compendium errata to make clear how folks could actually cast the nerveskitter spell in the first place:
The spell is missing some explanatory text the spell description. Insert this sentence after the first. "Unlike other immediate actions, you can cast this spell while flat-footed."
So, for example, the DM declares that an encounter begins and starts the combat music. The players and the DM arrange their miniatures on the table. Then, before the PCs make initiative checks, the wizard's player says, "Sivart takes an immediate action to cast nerveskitter on Ekaj." Initiative checks are made, and Ekaj reaps the benefit of Sivart's nerveskitter spell. When it's Sivart's turn, he'll be unable to take a swift action during it, having already taken an immediate action prior to cast the nerveskitter spell.