I'd say a+b
You have to use the weird die, otherwise it's not really a flaw (except for situations using 4 or more botch dice).
You could rule that the weird die is the only one left, but that seems like getting a bonus from a flaw. Could be interesting, but i'd say no.
You need to roll 3, then Weird Magic adds 1 to make it 4. Cautious Sorcerer then reduces by 3, to make it 1 botch die. The remaining botch die is the "weird" one.
II can see the argument for "a + b", but my counter-argument would be that "a + b" doesn't give the character the full benefit of his Virtue Cautious Sorcerer. It's better in such circumstances to err in favour of the character rather than against him, IMO.
The counter-counterargument is that the same reasoning applies when a character with Cautious Sorcereror (but not Weird Magic) faces a 3-botch-die situation. Only reducing it to one botch die doesn't give the character the "full benefit" of his Virtue. Or I guess it does, since that's how the Virtue's mechanics are defined: the full benefit changes in certain situations. The same applies here. (That being said, I can understand those who choose "b".)
To me, Weird Magic is a side bet on the usual game; it feels like it should just add on to whatever was going to happen anyway. Hence my preference for "a+b".
Say we had a situation where the character:
a) had a +2 modifier to botch die (say, trying to cast while walking a tight-rope, in a thunderstorm).
b) had the Cautious Sorcerer Virtue.
One could argue that:
"The default number of botch die is 1. Cautious Sorcerer reduces this by 3, which would make -2, but it cannot reduce below 1. So, the total modified by Cautious Sorcerer is 1. I then add the +2 situational modifier to make 3 botch die."
So, by being stupid about the order that I do operations, I have cleverly negated all the benefit of Cautious Sorcerer. So, you can always make Cautious Sorcerer totally useless by choosing the order of operations "correctly". This hardly seems to be the intent of the Virtue, but this is precisely what those who are arguing "a+b" is the correct interpretation are advocating.
The character has 1 botch die, Cautious Sorcerer, and Weird Magic.
Does the character get:
a) 1 "normal" botch die
b) 1 "weird" botch die
If you are arguing that the answer is "a + b", then you are saying that the Cautious Sorcerer Virtue does nothing. The answer has to be "b". (So, I change my mind, "a+b" is not just a different interpretation, "a+ b" is wrong).
No, I'm saying that Cautious Sorcerer reduces the character's Botch die by by 3, minimum of 1. Those arguing for "a+b" are saying that Cautious Sorcerer reduces the character's botch die by 3 minimum of 1 (or sometimes minimum of 2). Which isn't what the RAW says.
In the original example, if the character didn't have Weird Magic, then the answer would be a.
Think of this example: the character has 1 botch die and Cautious Sorceror, and no Weird Magic. Does the character get the botch die? Certainly, those are the rules. Are we saying that the Cautious Sorceror Virtue does nothing? Yes, in some situations. Therefore it is reasonable to think that there can be other situations where Cautious Sorceror does nothing. This new "a+b" example can be one of those other situations - there's nothing logically inconsistent about that.
Yes, it is. You "roll one extra botch die"... "botches from this die..." It's still a botch. It's just that the storyguide should use something more weird than dangerous if there wouldn't have been a botch without it. How do you know if that is the case? You track that die separately. Half the suggestions for typical botches can already be more weird than dangerous. Thus this particular part seems much more about flavor than mechanics to me.
Personally, a warping point comes from a magical botch, so you would get a warping point for this one, too.
Yes mechanically it is a botch but it is specifically meant to cause effects differently from that of a botch.
Adding to that, if you consider it as a normal botch die except from its specific effect, will you also have it affect the severity of a botch? If you get a botch on 2 dice and the weird, do you make the final botch effect according to what you would for 2 or 3 dice if the weird die wasnt there?
To me it would seem that you would have to use 3, if you´re also having it add warping, otherwise its inconsistant...
Also, if it adds warping and acts like an extra botch die mechanically, its just too "small" a flaw. It should very probably be a Major flaw then.