Hi :)
I have a question.

Let's say a PC faces a monster with a freaky reality-altering power... it can basically erase matter from existence, with just a glance, no physical contact needed.
(fans of the manga "JoJo" may recall a similar Stand power)
Maybe it doesn't even cause special mechanical effects; it just allows the monster to delete physical matter in the fiction.

The PC and the monster get into Simple Conflict ... the Monster wins, so the PC goes Extended.
Let's say that the PC goal is "I run away" while the Monster goes for "I capture you" ... or something like that.
The PC acts and describes how he tries to put distance between him and the creature ... rolls are made... etc.
Now the Monster acts and with his action he deletes one of the PC's arms ... rolls are made ... whatever ... etc.

My question is: can this happen?
Can a PC end up maimed in this way?
Is it OK for an effect to scar a PC so deeply just with a bit of narration?
Can the PC call for some rule or system mechanic to avoid being mutilated in this way?

Thanks ^_^

PS:
I forgot to clarify ... I'm talking about the Solar System ( obviously, but just to be totally and redundantly clear ^_^ )

I'm afraid I can't answer your question, but you have made me curious.  Which character in Jojo's are you talking about?  I actually recently read all of it and the weird powers and narrative flow are probably the biggest influence I have on my current RP project.  The powers get pretty ridiculous as the series moves on, but I can't actually remember anyone with straight up matter dismissal powers.  As unbeatable as powers seem to get in Jojo's Bizarre Adventure there's always some sort of wiggle room that the characters find and exploit.  The struggles get pretty abstract at times, but it's always there.

Jojo's is interesting because the characters CONSTANTLY pull shit that most GMs wouldn't allow their players to get away with.

Alessandro,
  Try applying this logic to your questions:
http://www.indie-rpgs.com/forum/index.php?topic=18620.0

  What I mean is, the narration of a scene is negotiable, unless:
1) A secret gives very specific narration constraints
2) It is the stakes of an Extended conflict

  So, to put it another way. If that back and forth was an Ability Check, then They can only lose their arm if that was Intent/Stakes that the player agreed to. And if it was rounds of an Extended Conflict, then they can only lose an arm as a result of a Secret. And if that is the end result of an Extended Conflict, then they can only lose an arm by losing to someone with those Intentions.

  Now, that is my understanding (which may be wrong) based on the book in vanilla mode.

  It sounds like you want to use very specific Genre/Tropes. If the group is on board for that, then you can do what you like of course.

  Does that make sense? I hope it helps.

My call would be that if the monster had this sort of power, then it may use it and inflict a maiming, unless the target extends the conflict or changes his intention in extended conflict to protect himself against it. Also note that if a character's intention is to "escape", it's pretty reasonable and at least charitable to read this as "escape without serious injury, such as losing an arm". We don't need to detail all expectations that characters have, after all; all conflict goals presumably include "and continue breathing" as a side clause even without having the player draft his goals carefully.

If there were reason to expect that the character would be OK with losing an arm (it's a robotic arm, say, and he can easily replace it), then the player would have a choice: will he allow the opposing declared action to go into resolution just like that, or will he declare a change of intent to revise his intention from "escape" into "escape without injury". This might seem nitpicky, of course, but it's actually quite valid in situations where the victim is unaware of the graveness of the situation: he just thought that he's trying to escape from a mad game, but now he realizes that it's actually also his continuing health that's on the line. The character pays for this hesitation and readjustment with a passive turn, and then continues the conflict with a revised understanding of the situation.

In general, focus on choices rather than strategy when it comes to theme-full adventure games such as the Solar System: the monster should not get to cripple a character because the player made a mistake at some step of the game; the crippling should be the result of a thematic choice - the character might choose to trade the arm for his freedom, for example.

Also in general, when it comes to character integrity, Solar System does not have a special protective field for characters: character health, sanity and self-integrity are all up for grabs in conflict just as easily as everything else. Read the part about setting stakes and propriety: the players may willingly limit themselves in what is appropriate for the genre and what is not, but the rules do not do this for them. This goes both ways, though; just like a character's body is not especially safe from getting mutilated in conflict, it's however just as safe as everything else the character stands for: you can't mutilate the character before the player gives in in the conflict or spends his Harm track against the outcome. Only broken character can be overcome against the player's will.

Finally, consider the results of allowing a character to get maimed: the mechanical results of this are powerfully constrained and known in advance, so a player may make the choice about allowing his character to lose a limb in full knowledge of the consequences. He might think that his character loses its viability in play if he loses an arm or leg, but that's the player's choice, not anybody else's.

Thanks for the answers, they were very helpful :)

THIS is the JoJo stand I was talking about : http://jjba.wikia.com/wiki/The_Hand

This interestingly flushes with some of the philosophy that has developed for me around dealing with physical disability issues in the Solar System mechanic in general. Often when I've encountered players who want to do things like this, they talk about building flaws for their characters as some sort of Secret. My philosophy on this is almost always that Secrets should be overwhelmingly positive--they can contain quirks and drawbacks, but they have to be a net gain to a character. As such, there are lots of flavorful ways to consider how this can be an asset, and thereby a Secret. Players who want to play deaf characters who can communicate via sign language that they can teach others, or read lips even at a distance, have benefits that are Secret-worthy. Someone who wants to play Zatoichi, the Blind Swordsman, should have a Secret which reflects just how capable he is despite his blindness--how he can hear amazing levels of detail, and fight by hearing alone--rather than just some sort of "negative Secret" which focuses only on his blindness.

I've sometimes tinkered with a Key of the Disability, which would work something like this:

Key of the Disability
You have some sort of impairment, which marks you as different and makes certain tasks harder for you. This might be difficulty moving, the loss of a sense, a missing limb, a psychological disorder, or anything else the player and StoryGuide can agree upon. In certain game settings, this might impose social difficulties as much as physical ones, as intolerant societies are not always forgiving of those who are marked as different. The responsibility for invoking this disability is purely on the player, NOT the StoryGuide, who can invoke this Key in situations where he deliberately fails ability checks or requests penalty dice for actions.
1 XP: Any time your disability comes up in a scene, marking you as different or making your everyday life more complex.
3 XP: Any time your disability gets in the way of something you really want.
Buyoff: Overcome your disability, so that it no longer negatively impacts your life.

We've had a blind pirate queen in one game, and a Stone Age hunter who was mauled in a mammoth hunt in another. I feel like it's been moderately effective, though I sometimes feel that the "responsibility for invoking on the Player side" aspect can be hard. Players don't always know when to invoke this, and there are times when I've set up scenes expecting the disability to be a major issue and found that the player hadn't even considered invoking it, as they were wrapped up in the drama of what they wanted rather than what might be stopping them. So often, players aren't responsible for creating their own antagonists or environmental impediments, so it's easy to let it slide in a scene.

-shadowcourt (aka Josh)