No, you still target the thing you can see with 100% certainty, even if that thing was not what you thought it was.
An example: the DM sets out the enemy on the square grid (or table or whatever). There are three creatures: a cleric, a fighter, and a wizard.
The wizard has cast mirror image on himself, the cleric has used an ability which allows him to create an illusion of a lion, and the fighter has activated a Hat of Disguise to make himself look like a damsel in distress.
That's the real situation. What do the PCs see?
They see (assuming no true seeing or other illusion-piercing magic) a guy in a starry pointed hat made up of four images (being familiar with the spell they know that one of these images is the real guy but that they cannot know which), a guy in full plate, warhammer, and shield with a prominent holy symbol, a damsel in distress, and a lion.
PC's turn. The archer chooses at target, and he does so with 100% certainty. He can choose to target the lion, the damsel, the tooled-up holy guy, or the guy with all the images. His choice of target is 100% certain, but that doesn't mean that he knows what his targets really are!
Now it's the PC wizard. He wants to cast either magic missile, fire bolt or hold monster, and he can choose from the same four targets (holy guy, lion, damsel, guy-with-the-images) and his choice is 100% certain even though he has no greater ability than the fighter in regards to knowing the truth about his targets.
Lets go through each spell choice versus each target. First, hold monster. This spell doesn't need to be accurately aimed in any way. He just has to be able to see his freely-chosen target. If he targets the cleric or the damsel/fighter then the spell works as usual subject to a saving throw. If he targets the guy with all the images' then that guy must make a save as normal. His images simply do not interfere with the PC's choice of target. If he targets the lion then the spell will have no effect because the illusory lion is not a legal target because it is not a creature. The spell gets cast, a slot is used, but it has no effect.
Now for magic missile. Let's say our PC has four missiles and he shoots one at the lion, one at the damsel, one at the armoured guy and one at the guy with all the images. Just like the other spell, this spell does not need to be precisely aimed, it automatically affects the chosen creature with no save. The missiles damage the fighter/damsel, the cleric, and the 'real' wizard, but does nothing to the illusory lion because it is not a valid target.
Now fire bolt: this requires actual aiming and an attack roll. Against the damsel/fighter or the cleric, a successful attack roll results in damage. Against the illusory lion, since the spell affects 'a creature or object' it is not a valid target. Against the guy with all the images, the PC wizard chooses to target that guy with 100% certainty, but then the spell kicks in and it may switch the target to an image instead.
Of course, conceptually, what is actually happening in the game world is that the fire bolt does need to be aimed accurately in order to affect the target (unlike the other two spells) so he aims at one of the images and hopes that this image happens to be the real guy. He might be right, he might be wrong, but he cannot know before the attack. The results of the attack do inform him if he was right or wrong, as a hit illusory image will wink out but the real guy will not.
So, choosing your target is 100% certain, without making you certain what your target really is.
