Hexproof doesn't actually mean anything on a spell, and that's by design.

Spells can have lifelink, so why not hexproof? It works in theory.

Spells can have hexproof too: for example, if you play a Slippery Bogle, it has hexproof on the stack while it's a spell. However, the rules do not give any special privilege to hexproof on a spell, only to hexproof on a permanent or player. So granting that ability to a spell won't do anything useful.

Oh, alright. Thanks. Card errata'd to "Target spell or permanent you control can't be the target of spells or abilities until end of turn."