So, after looking at this thread -> https://forum.atlas-games.com/t/charged-potions-thoughts-questions/10797/1 I got me wondering about something that I've considered sometimes, not only in Ars Magica but also other settings.
So, this has some ArM background, at least conceptually. The way Quaesitores use the Code of Hermes revolves around the idea that any magus who breaks the Code is also implicitly renouncing its protection. When a magus kills another magus, he's also implying that he's not willing to abide by the Code, and therefore, he's free for the kill without lawful consequences. Of course, this is just a convenient way to justify the validity of a punishment, since I doubt most magi would willingly drop the protection of the law, even if they break them.
So, what if this could be applied to magical resistances? The idea would be that certain actions taken by the target could allow for spells to bypass their MR without having to penetrate, even if they're not "technically" dropping them at all. In the thread I mentioned this was briefly addressed. Wouldn't drinking a potion be an action that by itself would imply that the target is willing to allow the effect to take place? That case in particular seems not proper to me, since anybody could be unaware of the fact that they're drinking a potion, and drinking something is an action too... common? to be used for this idea.
In the other hand, we could consider more interesting possibilities. A ward that bypasses the MR of anybody who willingly crosses a barrier, for example, by implying, by doing so, that he's allowing for the consequences of said action? That would be kinda broken. I can think of a good example right now, but I think this is an interesting subject to explore.
The only thing that I didn't like about MRs were the fact that they simplify defending against magic too much. While you can be "creative" by indirect means, like dropping a mundane tree on top of your target, opening a hole under their feet, hiding traps with illusions so they fall into them or just destroying all air around them, it seems to me a bit too easy. No matter how strong a person is, we aren't anime characters whose muscles get so ripped that bullets and swords end not being able to hurt us, or being able to endure flames, extreme cold or falling from great heights. A well-aimed toothpick can kill a strongman just as easily than if he was a baby. Of course, experience and training allow to actively avoid these things, but not passively. A wizard dumb enough to not to check if his flask had been tampered with, can be easily killed with a drop of a multitude of poisons, but he could literally bathe and bask in spells aimed to damage him without harm even though those spells would probably be several times more powerful than what it would be needed to create those poisons.
I understand that this is used to simplify the rules so our characters don't spend half their lives creating wards against every possible way of harm. But it's not like you'd invest all of your time learning how to deal with most dangers by mundane means either, even though you probably could and nothing prevents you from doing so.