The problem isn't with Zero's mods.
It's in the "Forbidden Spells" mod from Kuttoe. But it seems very specific which is why I didn't catch it until I grabbed Zero's mods.
If...
You have a spellcaster with an Ancient Bloodline trait, and you visit the vendor in caster's alley that sells familiars and spellbooks, and you have:
ForbiddenSpells
- then you will start getting script errors, after you have saved, quit, and re-entered the game at least once after BOTHJ that mod was added and you've visited the alley. But I think the error can occur the first time you talk to the vendor if the other steps are already true.
Once these errors begin, the save file is bad - deleting the mod won't fix it.
A spellcaster that lacks at least Ancient Bloodline (I didn't test other bloodlines) won't trigger this error - so this can be hiding in your save for a while before blowing up on you, but if you delete the mod before approaching with a bloodline traited sim, your save will be recoverable.
Forbidden spells comes with 5 package files and a script file - I've been unable to isolate exactly which of them is the issue.
My theory is that it could be any but the bees one - as they mess with the costs of spells and scan your age - and these all seem like things that a bloodline would also care about. The bees one is probably OK because it just sends a swarm of bees, but to be safe I removed the whole pack.
My spellcaster also has the 'Good' trait, and all of these spells gives a mood penalty to good sims - so maybe that has broken since it came out and it has nothing to do with a bloodline. That's one more test to isolate - but would still be a reason to not use the mod as the 'good' trait is from Maxis, and this messes up the vendor even if your good sim has no intention of learning those spells.
On Kuttoe's website it's noted that they abandoned the idea of working on spells some time back because the spell injector was unstable and abandoned - but I've not yet seen problem with custom spells from other authors.
This feels like a high risk error - the kind that could hide in your save file for a while before you catch it, and which I've yet to find a way to clean out of a save once it's there. Granted I can't imagine why I'd go to caster's alley and NOT talk to that one vendor the not come back for some time... but it could happen.
I'm just glad my first instinct on making an ancient bloodline "harry potter knockoff sim" was to run over there and buy a familiar, and catch this before I'd spent days on a new sim.