This question already has an answer here:
How would things resolve when targeting a spell, like magic missile, at an illusory creature?
For spells like Fireball, Lightning Bolt, etc. I think it's fairly clear the caster fires the spell off at the location they think there's a creature in, and it's up to the person controlling the illusion to decide how it reacts. Those spells don't need to be aimed at a creature at all, so the fact there isn't one there is irrelevant.
I'm asking about magic missile in particular because there's no attack roll required, and it automatically hits whatever it's aimed at. In fact WotC have confirmed on Twitter that it bypasses mirror image for that reason - no attack roll, it hits the creature you aimed it at. But what if you're aiming at a creature that doesn't exist at all?
Does the spell fizzle and fail to have any effect at all - since you haven't targeted it correctly? Does it fire off and seem to hit the illusion? That seems wrong to me, since you can't normally make a magic missile hit non-creature targets.
RAW seems to suggest that the spell would fail because you hadn't targeted it correctly - which means it's a great way to reveal illusions. But is there anything that suggests otherwise?
This is a similar question to the linked one, but I'm not asking only about Magic Missile. The question is broader, and is about any/all spells that are required to be targeted at a 'creature' or similar, where an illusion wouldn't fulfill that requirement, or indeed any other condition that led a caster to believe a valid target was present for a spell, when that wasn't the case.
Other examples include Healing Word, Polymorph, and so on. Is the caster aware that the spell has failed? I note the answer on the linked question suggests two possibilities - either the spell goes off, or the caster fails to target the illusion - which I suppose suggests there's no official ruling addressing this.