Yes, the common example is usually trying to cast charm person on a dog. You can cast the spell, it just wouldn't do anything. The spell is still used. Similarly, maybe you cast charm person on a doppleganger (because it looks like a human or something); as a monstrous humanoid, it would not affect it.
One more example would be magic missile. You can cast it a door, it just wouldn't do anything. The missiles would hit the door and not affect it at all. Yes, the target line says creatures:
Magic missile wrote:
Targets up to five creatures, no two of which can be more than 15 ft. apart
but that doesn't stop you from casting it. Otherwise, if you couldn't cast it at an object, they wouldn't need the line:
Quote:
Objects are not damaged by the spell.
Since the spell doesn't miss, there really isn't any reason to have this (Other than the rare case of an invisible wall between you and the target I suppose.) Why would you cast it at a door? Maybe you thought it was a mimic or something.
Either way, in your case, the spell is used. It fails and the player doesn't know why (or even that it failed); unlike when the target makes its save against a targeted spell, since the save never happened. For all they know, it could be an invalid target, it could have SR, it could be standing in a globe of invulnerability or a protection spell, etc. (Out-of-character, he may know you didn't ask for a Spell Resistance roll, of course, but that's OOC.) In-character, as far as he knows the spell worked until he observes something that tells him it doesn't, ie. he tries to have the illusion do something and it (most likely) doesn't react in a way a charmed creature would (unless the bad guy was aware of what was attempted and was able to control the illusion just to mess with him).
