Say a magus is using a ReCo fast flying spell, and for whatever reason (failed a Finesse roll, distracted by a demon or Fairy, unexpected fog, etc) slams into a solid surface (Wizard tower, cliff, etc) at what should be bone shattering speed.
Would the mage's Parma Magica protect them?
Part of my brain is arguing magical propulsion is blocked by Parma, while the other part is saying they are being propelled by magic into a mundane hazard, not the mundane hazard being magically propelled into them.
Unclear which argument to listen to.
Magical propulsion affecting them would have already have had to either penetrate or been under their parma to begin with (personal range). Once a spell affects them its effects work on them, for good or ill.
But it does raise another hypothetical:
Rather than slamming into a solid wall, what happens if they fly into another mage, also with Parma up?
The stationary mage should take no damage (assuming insufficient Penetration), but the flying maga? Would they take damage from the sudden stop, but technically no physical impact?
You are thinking in terms of modern physics, which do not exist in Mystic Europe. Anything protected by Magic Resistance is immune to the effects of non-penetrating magic. However that protection does not extend to things beyond them (ignoring the ability to share Parma).
The flying magus will take damage since the magic penetrated their MR (otherwise they wouldn't be flying), while the stationary one will not if the spell is not powerful enough to penetrate theirs.
I think under MR rules this is very up in the air and could go either way depending on how you view MR. Troy made a good case for no damage but would the spell trying to cross MR keep working except where it was trying to cross (flattening the magus against the MR as in Troy’s take above) or does the act of trying to cross the MR end the effect completely which would not flatten the first magus against the other’s MR but might drop the magus to the ground which could cause damage or not, depending on the fall.
I think asking for coherence here, as the primary concern, seems like it may be misunderstanding the magical-medieval physics the game is based on and basing it more on making it simple and more friendly to being shoehorned into a ruleset than "how that world is supposed to work."
As I've interpreted it (and would run it in my campaign, where I do have a naughty flying magus all the time - he's been seen by mundane and will get hunted as an unnatural flying witch shortly), the magus definitely will get any and all damage from running into objects - magical or mundane. If he flies into something with Might, MR or Parma - his flying spell would have to penetrate as usual to damage it/them.
The magus is essentially using himself as an unnaturally moving projectile... Now if he just launched himself in a natural ballistic arc, he'd damage the target without having to penetrate.
If he wants to not get the damage, he can use other spells to solve that. The flying spell wont by itself.
It's looking classic pink dot problem.
If parma protects in such a situation, just before impact with any fall, magic a pink dot and you've successfully flown by throwing yourself at the ground and missing.
I consider when in doubt, go with the rule interpretation that is less open to abuse.
It'd still be coherent for a medieval understanding. You'd have to invent how it extends for magic.
If hitting the stationary magus does no damage, a paper plane thrown with magic would also take no damage under the same circumstances. Either way, the answer remains coherent.
You might even come up with other ways to fit medieval thinking. But if you cannot, you are moving from "what actually happened" to "the myth it became". The whole inconsistent story is the legend told 20 years later.
They were asking for coherence between two different situations, falling onto a magical bridge and not being protected by ones own Parma having to have the same effect on the moving magus as a different event, flying into parma and the spell stopping your motion due to MR.
And, yes, your airplane result is what I mean, it doesn’t crush the paper airplane because the spell ends and no more force is applied immediately. Now I did have the thought that maybe that would not be desirable because it would behave differently with ReCo, for instance, than indirect flying spells like the one that uses wind to propel you and your motion is not magical though that was always the case with whether you knock over the magus you run into or not or his parma stops you completely.