In a relaxed situation, if you do something wrong you can start over. This is canon as otherwise you could not practise spellmastery for rituals without using vis.
Doesn´t feel right? What doesn´t feel right is to make healing a not quite 1/10 (3.4% absolute minimum) risk of spectacular failure. Or that a similar amount of towers raised by magic will be disasters. Or that rituals for better harvests will have a notable risk of causing a wasteland(or a jungle) instead. Or creation of a resource...
Or making it drastically easier for magi to end up in twilight. THAT and many things like, THAT does not feel "right". Nor have i seen anything what so ever in the game fluff that even mention it happening ever. Why oh why might that be do you think?
By RAW that is clearly not so. If it was they wouldn´t bother to very specifically separate between the different concepts in some cases. Nor specify that stress die does not equal botch risk.
So here's a case where a stress die from Spell Mastery is clearly not beneficial, if going by a strict reading of the RAW.
If you have Magic Addiction, spell mastery is not desirable. The text for Spell Mastery clearly states that the die used is a stress die, even if the caster is relaxed.
The text for Magic Addiction states: "Whenever you use a stress die in spellcasting, you must, whether or not the spell succeeds, make an Intelligence + Concentration stress roll, against an Ease Factor of half the level of the spell"
So, by RAW, if you have Magic Addiction and have mastered a spell, you have to check for succumbing to addiction. However, I don't believe this is the intent of the authors, I use it here to point out that authors sometimes write something that is clearly against their intent. That it has bearing on Spell Mastery and the use of the stress die just lends a bit more evidence that the term stress die used in relation to the Ritual Magic section is unclear as to the intent of the authors.
It says under mastery that if the magus is relaxed they do not roll botch dice "even in a non-magic aura or when using vis", which to me suggests that if before considering the aura or vis the botch dice are reduced to 0 there are no botch dice, even for ritual spells.
Sure the authors might not have considered this precise combination. On the other hand, it doesn't really seem incompatible with what Magic Addiction and Spell Mastery could mean to the character.
To master a spell you must have obsessively, repeatedly cast it (or at least similarly obsessively studied it), which sounds rather like an addiction...so it doesn't seem out of character for a mastered spell to trigger a related addiction episode. In real life, people who are addicted to things tend to have "mastered" any "skills" that might be related to that addiction.
If it's possible that this precise combination wasn't considered (Magic Theory and Spell Mastery combining), then it's certainly possible that they missed other combinations.
For what it's worth, a stress die on a relaxed spell that is mastered is a stress die without a chance of botch, and I think that that factor is rather important. I stipulate to your comments about addiction, generally, though.
I could actually see this being useful- have the addict master a utility spells from covenants (to change trees into lumber for example)- he casts it once, fails the addiction check, and will happily continue to cast it all day long to feed his addiction. You now have a magical lumber mill. Meanwhile the poor addict is trying to explode the dice to see just how far he can push making lumber..
Most of the Craft spells are low level enough to not have to worry too much about Magic Addiction. 20th level spells even have a reasonable chance of being controlled. It's an EF of 10 versus of Int + Concentration + die roll. That's not too hard to get. It's everything above that 20th level that gets interesting, with 35th level spells often having a huge risk of spiraling into unconsciousness...