What happens when you accidentally choose an invalid target for a spell?

[Tiene]

The section under "targets" here says that if you, for example, target a vampire that you erroneously thought was a living creature and the spell requires a living creature, it fails to target the creature. But what does that mean? Is the spell slot used? Is the action spent?

[Val'bryn2]

Yes.

[Cordell Kintner]

You cast the spell normally, and it has no effect. This is different from being "immune" to it, as there could be some things that can bypass immunity.

[Darksol the Painbringer]

I don't think that's what that entry means. I'd like to think that's what that means, but if that's the case, the entry could have just said "it has no effect," which creates this desired outcome, just like being immune does. But it doesn't say that. It says your spell fails to target the creature, which means you have to target something else that is valid. It raises a question of what to do when there are no other valid targets, though, but I don't think the book really thought that situation through.

To me, it has a different result, which is that the spell is expended on a different target, because it failed to be a valid target to begin with, not that the spell is expended with no effect.

[Jared Walter 356]

Darksol the Painbringer wrote:

I don't think that's what that entry means. I'd like to think that's what that means, but if that's the case, the entry could have just said "it has no effect," which creates this desired outcome, just like being immune does. But it doesn't say that. It says your spell fails to target the creature, which means you have to target something else that is valid. It raises a question of what to do when there are no other valid targets, though, but I don't think the book really thought that situation through.

To me, it has a different result, which is that the spell is expended on a different target, because it failed to be a valid target to begin with, not that the spell is expended with no effect.

Personally this reads differently to me. The words "the spell fails to target that creature" reads to me as proofing for multiple target spells. Ie, if I target 4 creatures and only three of them are valid, it affects those three but not the fourth. Especially when combined with this text from the previous paragraph "If you fail to target a particular creature, this doesn’t change how the spell affects any other targets the spell might have." describing how spells can sometimes target creatures you can't see.

This strikes me as the same language used for hidden creatures and the flat DC check to target them. "A creature you're hidden from is flat-footed to you, and it must succeed at a DC 11 flat check when targeting you with an attack, spell, or other effect or it fails to affect you. Area effects aren't subject to this flat check."

I read this as the spell is expended without affect the target.

[Cordell Kintner]

The same section in Spells mentions that you can attempt to target a creature you can't see, and if you miss the spell fails to target that creature. It doesn't mean it has to jump to a valid target. The spell is already cast then fails to target, so in the end it has no effect.