This has never come up in any games I've run, oddy enough. As The Wizard's Sidestep causes the image of the caster to appear elsewhere, what effect does that have on spells cast by other magi that target him or her? Obviously a spell that has to be aimed is likely to hit the illusion. But otherwise, no spell can affect a target the caster can't sense. In this case the caster is only sensing the species (in the Ars Magica sense), rather than the actual magus who's cast The Wizard's Sidestep on himself. So would a spell like Pilum of Fire hit the target or just the illusion of the target?
That is odd yes.. you'd think it'd come up quicker.. had never thought about it myself really.. as you say the aimed spell wouldn't be a bother.. but I think non aimed spells would still hit the magus.. as all you're required to do is sense him (which can be done via various magic means too after all).
I tend to think of the various senses caused by species as just very finite duration arcane connections for simplicity sake.. so if you're seeing the mage, you can target him because they're still actually his species, same way that if you cast the spell and they fast cast teleport 5 paces away.. it'll still hit them.
But that would mean that you would have to avoid being fooled by the illusion before casting anyway, and then the illusion doesnt matter at all.
If you´re convinced the target is standing at X, you´re not going to aim at Y(which you cant see). Magic wont do any thinking for you.
Spellcaster decides where the spell hits.
That leads to some less than preferable problems...
For one thing, it suddenly means you can target someone out of sight, and persons hidden. If their image IS within sight.
And if the image is within sight(and within Voice range), but the real person is outside of Voice range and a Voice range spell is used against it...
Then how do you handle it?
And if you can hear a person but not see, then you can now suddenly target that person with a Sight range spell if we extend your thinking? And yes, no matter how silly this sounds, it still is a logical extension of the idea even if its farfetched.
And why do you not have to be fooled by the illusion to cast a spell at them?.. it's not aimed, as far as the magic is concerned, you're targetting that man, and that man is in range, even if he's 5 paces away.
If the spellcaster is deciding where it hits.. then why isn't finesse rolled against dodge? Because most people would dive out the way if a fireball was heading at them.
As for the latter points.. for you to hit someone with a spell, they have to a) be sensed, and b) be in your range for the current spell. Someone can be the other side of a wall and if you can sense them through whatever magic means, you can still target them. If you can see their image but they aren't -actually- in sight range.. then no, it won't work.. they're not actually in range, but I imagine you'll have learned they were out of range at least, so something funny is up.
And for the last one.. see above.. if someone isn't in line of sight, then your sight range isn't going to do much for you.
Being able to move your image isn't meant to be an insane advantage, but a careful person could probably hide themselves away somewhere that couldn't be easily targetted while doing so, if they have enough range on the ReIm
IMS as long as you sense the magus you can target it. If the spell is aimed you fail, but if it is not, you hit the magus. A punny level 10 spell cannot prevent this: it can help in mundane defence, bu against magic you'd need to try harder t protect yourself.
Xavi
But well, if the magus is behind a corner while his displaced image isn't... ?
Based on the logic so far in the thread (you can target someone with magic even if the image you see is displaced), any spell that targets the magus personally (like Wound that weeps) would hit, while a spell that creates/flings something at the magus (like Pillum of Flame or The crystal dart) would hit the illusion.
I'd suggest that a lot depends on the range of the spell and whether it is aimed or not. A spell that is aimed or one that requires visual targeting (such as an Eye or Sight range spell) is likely to hit the illusion or simply miss altogether. On the other hand, a Voice range spell would be likely to hit its target, as long as the voice reaches him or her, even if the pesky magus in question is hiding around a corner, and by the same token, a spell cast at Arcane Connection should have no problems...
Just my two pennies.
(ulf, pilum of fire does not require targeting: you immolate the target in place, without "flinging" anything at him. Just FYI 
)
The ambiguity of the word "target"... For a pilum of fire, the target is actually the fire itself, and that will allways be exactly where you want it. Offcourse, that does mean that if you aim for empty air, then you will hit empty air - even if there is a nice illusion there...
The reason (at least as far as I can understand it) that you do not have a targeting roll for this is that you can simply guide your fire all the way to the target - so no matter how much you dodge, the flame will still hit.
Personnaly I'd lean toward the notion that voice ranged spells that still target 'individual' still have to 'correctly' target an individual and that as such, the illusion is still a helpful defense.
That being said, I also think it's ok for a magus to target an 'individual' spot where he suspects the magus may be, unless the form of the spell restricts the validity of the target. That is, targeting a spot for a pilum of fire would be fine,... but targeting a spot for a corpus spell (where the corpus is in there because that is the target) would not. The spell needs a valid target in order to 'initiate'. I'm not sure if I'd let a corpus spell attempt happen and let it 'fizzle' - fail to initiate owing to there not being a valid target.
I like this interpretation. Adds a bit of tactics to spell design, if you hunt the most dangerous prey of all...
"On the other hand, a Voice range spell would be likely to hit its target, as long as the voice reaches him or her"
I have a problem with this in that a voice range spell with an 'individual' target means that you still have to 'select' a target. Unless the spell indiscriminately selects all targets within range, there must be a mechanism by which to decide 'what get's hit' and 'what doesn't'. What would be the justification that 'the right spot' got hit? How was the discrimination decided upon mechanically?
Good question. Way back when we first conceived Voice as a range (for B&S), it was specified that you had to name the target in two ways. That has gone by the wayside with the incorporation of Voice range into the Hermetic spell guidelines, but you could still house-rule it as a requirement to make sure that the spell hit the right person.
"Creo Ignem. I target Ignus of Flambeau, the smelly magus with red robes."
Or:
"Creo Ignem. I target the smelly magus with red robes, the one who is hiding his pathetic craven self behind this paltry illusion."
I don't think a verbal description of a 'smelly spot' would be adequate for targeting purposes on a non-aimed spell. I might require a finesse roll in such a case. One of the things that Mutants and Masterminds did that I thought was interesting and useful, was distinguish between 'targeting' and 'non-targeting' senses. So sight was a targeting sense for most humans, smell wasn't. Smell might be a targeting sense for most canines.
Where an "eye" spell requires the sense, I'd rule it can't hit invisible eyes at all, but could hit the illusion. I'd rule that a targeting spell trying to hit the real target does so with some kind of awareness roll determining the penalty on the shot. On non-targeted 'auto-hit' spells, where the target guidelines seem to make it possible, I'd let people target the 'space' in hopes that they'd hit. I'm thinking now that it would be a similar awareness roll. Logical arguments would help the ease factor of the roll I think. That is, if you know you're looking for an invisible mage's shadow, it will be easier to detect the mage's location than if you didn't know that trick - with the amount by which it helps being determined by environmental factors.
In short i would say...
The spell hits wherever the caster thinks it should hit. If he thinks the illusion is his real target, then the spell hits the illusion. If the caster knows its an illusion but dont know where the real target is, he can guess and hit the spot where he thinks the target really is, but casting at the illusion will only hit the illusion.
I dont accept selfcorrecting, "intelligent" magic. If it did, then for one thing there would never be any risk of botching. Ever.
I concurr with Direwolf.
Spells like PoF are automatically drawn to their target, if their target is an illusion then that is what they strike.
Well, I think the targeting-by-voice is too stylish a concept to leave by the wayside. It opens the door to just as stylish botches.
I can actually imagine the Voice range as having been invented by magi specifically as a counter to invisibility or illusion spells. However, I would still insist that the magus has at least to know that the enemy is there and be able to identify him or her in some way; that prevents magi from just casting blind into an empty room in case anyone is there. Of course, magic that will seek any available target would be a great candidate for a Hermetic breakthrough...
Bear in mind that a magus targeted by a Voice range spell still gets Magic Resistance. It's still not as effective as using an aimed, indirect spell (like dropping a mundane boulder on someone).
But that's just my opinion. 
I think you need to be able to accurately discern a target in order to affect it. If you unknowingly cast a spell on the displaced image of a magus, then it takes effect on the image, not the magus. You don't need to necessarily see the magus directly, but you would need to get a "fix" on him, either from his shadow, displaced undergrowth, the sounds he makes, use of an InCo spell, or some such.
For a spell with R:Arcane, I'd also say you could use the image as an Arcane Connection to the magus.