My first observation is that errata as you suggest would require us to delete The Tireless Flight entirely. It assumes that Unconsciousness, at least, is suppressed by this effect, and the spell makes no sense at all if it is not. That's a big change to make in errata, and we need a really good reason to do so.
I'm not actually very concerned about the edge cases. Conjure the Expendable Berserker is going to be fairly high level, and if you can cast that with enough Penetration to affect a dragon, you should be able to defeat the creature. It is also part of the point of Ars Magica to allow player characters to do absurdly powerful things by clever application of the magic rules — there is a whole chapter in Transforming Mythic Europe on building your own island, and with careful design you could do that with a group of magi fresh out of apprenticeship.
The idea that you could just keep these spells up indefinitely and thus not need to worry about Fatigue at all is a problem. (Certamen, less so. It is a formalised duelling system, and using these spells would obviously be cheating, so the certamen would not be binding, or the cheat would be deemed to have lost.)
Something needs to be done, but I am still thinking about what.
How about a wound build up? Instead of getting 223 Light Wounds, make it X light wounds, X Medium Wounds, X Heavy Wounds, Incap or Death (at which point the effect ends - you've pushed yourself too far). There may be some calibration to do as to how many wounds of which levels before the next wound upgrades.
I'm not sure this is much of a problem. If it doesn't shut off, you start taking Light Wounds. Eventually you have to worry about them healing v. worsening. And there are other issues as suggested in The Tireless Flight, where it's pointed out that being able to cancel it is good so you can start recovering. I've worked with variants of this spell for a long time and have never seen big problems arise except when people decide spells should be arbitrarily allowed in Certamen. In the few cases where it could be really messy, those special cases have their own rules for going too far that can override these rules (e.g. Life-Linked Spontaneous Magic).
refer to the (p.101) explanation of Mutantum Spells in the spell text, thereby explicitly tell readers that there is no version of The Tireless Flight without Mutantum Magic,
and then explicitly exclude from the spell's effect Fatigue and wound from combat and Certamen.
After all, this is a Mercere and Mutantum special spell.
(2) If there a spell similar to The Tireless Flight in some other book, neuter it in a similar fashion.
The Mutantes design spells that have unusual variations on standard magic built into their effects, and only characters with the power to perform that kind of magic due to an appropriate Virtue can cast them.
So spell out this appropriate Virtue in The Tireless Flight as Mutantum Magic, and that there is no other version of this spell.
Here's one of many solutions to that:
Using The Tireless Flight requires a continuous specific Mutantum type of Concentration to keep Fatigue transformed to Light Wounds.
Normal Concentration anyway can break in combat or Certamen. The specific Concentration needed for The Tireless Flight certainly breaks in that case.
EDIT: 'Flight' in The Tireless Flight comes from fleeing, not flying.
It does raise the question of should we do something about a ridiculous amount of light wounds? I've thought a rule of the 10th light wound is considered a moderate wound is a good option.
In combat against all but crazy large creatures, the chance of 10 light wounds occuring prior to incapacitation, is rare, and it reduces the ability to exploit some spells.
Strategically quoting a small snippet to leave out exactly what those variations are does not make your case at all. Those variations are spelled out quite explicitly. I recently saw someone elsewhere do exactly this same misleading partial quote to make the same false case.
@callen: No matter what you wish to argue, the change I propose for The Tireless Flight together with my quote from HoH:TL p.101 makes sure even to very sloppy readers, that there is no other version of this spell without Mutantum Magic.
I understand that. What I said above is still true: this would require changing the Mutantum Magic virtue as well. Read what I first replied to say. I didn't say it wouldn't work. I pointed out that there would need to be this extra change since you're now adding something new to Mutantum Magic.
A simpler solution, in my mind at least, which does not change the rules to suggest an equivalency in accumulated wounds of different categories, is to addendum the text such that every time a light wound is gained from exceeding the unconscious level it requires an immediate roll, , for the possible worsening of all other wounds previously acquire during this casting of the spell, specifying that recasting without first suffering the result of unconsciousness is considered to be the same spell.
I would note two things here.
First, The Tireless Flight still does make sense in my view. I do not think it assumes that Unconsciousness is suppressed. There are cases in which you can lose multiple fatigue levels at once. So the spell would make you feel no fatigue, but if you pushed yourself beyond the limit (by being already Weary and taking three fatigue levels) you'd get hurt. It might need a little editing for clarity, but it would still be useful.
Second, I think that since The Tireless Flight and Endurance of the Berserkers use the same guideline, they should have the same mechanics (other than as for RDT and Mutantum stuff). So it's actually a good thing if The Tireless Flight gets reworked a bit.
I'd just note that a magically conjured grog can whack a dragon with a mundane sword without needing to penetrate. And the problem is that this also allows the greenest (or most wizened) non-magical grog to do the same, as long as one is willing to a) lose that grog or b) spend 6 pawns of vis to heal him at the end of the spell.
And I would not call a level 20-25 spell "fairly high level*... though that's obviously just me.
Finally, sure, Hermetic Magic should allow you to do absurdly powerful stuff. But if you allow "invincible berserker grogs" then almost all other combat magic concepts become vastly inferior, which impoverishes the game.
There are two problems with that. First, the idea that fatigue-magic (actually fatigue-restoring, rather than fatigue-preventing, but hey) is admissible in Certamen is part of one of the hooks in HMRE (about Folk Witches brewing fatigue-restoring potions). So either that has to be rewritten, or some complex line must be drawn about what's admissible and what's not.
Second, if certamen requires scrying on the contestants to make sure there's no magic going on, it becomes much less usable (and besides, again, where do you draw the line?). I'd much rather have a system where everything on yourself is in principle ok (though it can be negotiated away) so there's no need to scry, or have a third party scry, your opponent.
To be honest, the impact on certamen of fatigue management spells doesn't bother me too much - because I think these sort of things only generate tribunal stories and possible peripheral code developments. Having imperfect laws that don't deal with every situation until you cross the bridge and someone decides to make that an issue is a good thing, story-wise, and actually fairly normal. The idea that you can ignore fatigue indefinitely or accumulate an endless number of light wounds, to be cured in a single ritual, that's different, as that seems a weakness of the mechanics rather than a story hook.
The spell is tethered so that the target can control the effect, allowing her to stop concentrating when she has reached her destination and has time to recover
This does not mean that you can't recover fatigue / heal while under the effect of the spell. It says the recipient can choose when to allow the spell to lapse - rather than being hit by that at an inconvenient time. If you assume the effect does prevent healing/recovering fatigue while active, that should be spelled clearly in the corebook where the guideline is introduced.
I didn't say you cannot recover Fatigue. I said there are "other issues." You're incorrectly reading what I said. Consider trying to eat enough not to run into trouble when you're never hungry. Considering roughly that is a major health issue in sports in the real world, I don't see that it wouldn't be an issue. Consider trying to get a full night's sleep to recover long-term Fatigue when you're never tired.
They do have problems when the wounded party does not generate defense totals. A heavily, heavily wounded character can be more easily kiled by a child's kick than by an Incantation of Lightning. But other than that, they are quite clever, so much that I often wondered why they were not ported over to Fatigue.