I made a dumb joke in another thread but it prompted a (slightly?) more serious idea - is there a flaw that works like magical focus, but with penalties instead of bonuses? So, for example, as a minor flaw I could cast Creo Mentem like normal most of the time but there's one specific emotion that I can't do at all - or for a major equivalent, I could do normal CrMe to put thoughts in people's heads but never emotions.
A flaw has to limit you. Oh no, In my magi build with an Auram deficiency, he also can never use weather spells. Free -1 flaw is what that is.
There would be some caveats I would put on this flaw, for it to be possible. It must be in the highest technique/form combination the magi possesses, and if at any time this is not the case, a -1 flaw that would be relevant is gained.
Since it would be trivial to avoid the need for anything less than Form-breadth it would be a Flaw with no downside. So, there's no such Flaw. Some of the MMF possibilities -- Healing, Weather, Disguise -- may come close but still easily worked around for minimal downside.
There are the Flaws of Deficient Technique & Deficient Form which are broad enough that any magi are hindered by them. Incompatible Arts likewise.
Oh yeah, I didn't think about the "will rarely be an issue" aspect of it, that makes sense.
It's a minor shame that it doesn't work because the idea is great. As @Hyalus says you can sort of do it, and still hinder the character, with other flaws but they don't give quite the same feeling of something that the mage is just bad at.
In the right group I might accept a MMF flipped into a flaw as long as everyone in the saga knew that meant the story would feature whatever that flaw was such that if spellcasting issues were being avoided it would become a social flaw (reputation?) as other people noticed that mage working around or having trouble with specific things.
Deleterious Circumstances could fit the bill, if you allow "target is a bird"
Deleterious Circumstances 'Target does not have active Magical Resistance'.
Seriously though; a character can work around their restrictions (and play to their strengths) pretty easily -- even a very generous storyguide might only allow the area of a 'Major Magical Focus' as a minor flaw.
I would agree with @OakRover, you could see a major magical **un-**focus as a minor flaw. It is much easier to work around a restriction, than to capitalize on a benefit. Deleterious Circumstances could easily be used as @Tugdual mentioned, though I would also mention Lesser Malediction as a possibility.
Even without making up new flaws, it's still important to check the list for non-impactful flaws in character creation. While i am sure that none of you fine magi would be so ill-mannared as to make a character with Poor Presence (x2) + Disfigured + Blatant Gift + Odious Personal Habit (rude to mundanes)....
Perhaps we should House Rule zero pt Flaws - magical quirks that almost have no affect in normal circumstances but adds to character richness.
Deletorius Circumstances against Birds. 
aren't those the negative(or positive) twilight scars? ie, what the SG wants?
It's probably good to zoom out when we're looking for operative inverses of Virtues and Flaws.
Magical foci-- major and minor --are really strong, and while our main focus while using them is typically, "Ah great, my [_______] total is half again what it was;" The virtue's function is that it doubles the score of the lowest art in the total. So really the virtue has two aspects: either making you really strong in the area of your specialization or making you capable anywhere in the focus where you aren't really statted out.
The virtue is supposed to get you into the habit of designing effects around the focus and interacting with the world through the lens of that focus. This means that while you're specializing in a material sense, you're enabled to approach a broad assortment of effects through that specialization, e.g.; I don't use "The Invisible Porter (Re:Te)" I use "A fuck-load of wind and finesse (Cr;Re:Au)" because I have Major Focus(Wind). Or, in the same case, I don't use "Grip of The Choking Hand (Pe:Co)," I instead "Destroy the wind in and around his body (Pe;Re:Au)"
So, in effect, foci are less about facilitating specialization-- anybody can specialize --and more about expanding the avenues of a specialization. The exchange for which is just being a little more predictable and probably a little hampered by the complications that arise from stepping on the toes of other forms.
So if we want the functional opposite of a focus, we don't want something that behaves as a focus with negative values. "When you cast a spell to Create Anger (CrMe), you have to subtract your highest art from the total." We want something that cripples your specialization, limiting it.
The general shape of that would probably be something more like
Myopic Talent (Major/Minor Hermetic Flaw)
Choose a field of Major or Minor Magical Focus. When using the Form(s) or Technique and Form associated with that Focus, your totals are halved unless you are working within the Focus you have chosen, in which case your totals function as normal. If you choose Creating Wind as a minor Flaw, your totals will be halved when Creating Rain or Lightning. If you choose Necromancy as a major Flaw, any totals featuring Corpus, Mentem, or Animal are halved if they do not pertain to The Dead. This Flaw does not need to correspond to a Magical Focus and one can possess a Magical Focus in something else entirely-- even if the totals of that Focus might be halved by this Flaw.
In the best case scenario, this basically takes your magus from being "Guy that is pretty good at X" to "Guy who can only do X."
That's an interesting way of looking at it. I like it but I feel that there is room both for "Guy who can only do X" and "Guy who is noticeably bad at X" as interpretations of a magical focus flaw.
Necromancy seems like a good example. It is very limitative.
Contrast to converting MMF (Animal:Bird), which turns to a weaker Flaw than Deficient Form (Animal). It's still worth a minor Flaw just for the flavor.
OTOH, you cannot adjucate any value other than none to a converted MMF (Corpus:Men). So MMF can become anything from major to no Flaw at all.
The good thing is that mMF are more restrictive, which means that most inverse can be adjucated as minor Flaws.
Could you give some more details? I'm not sure why Men is so difficult to adjudicate.
I really like your way of analyzing the case of what is a focus in game. This myopic flaw has a lot of flavor to it. I may add it as a greater malediction in my own games.
This doesn't compare well to existing flaws for balance. Deficient Form does this, for a Minor Flaw, applicable to an entire form whereas a Major Focus usually corresponds to less than en entire Form, for a Major Virtue (Major Flaw here, if I follow you). So even giving a Minor Flaw for your Myopic Talent covering a Major Magical Focus is too much except for niche cases like Necromancy which cover several forms. But Myopic Talent - Rain, for example, should never be a flaw, otherwise no one will ever take Deficient Auram.
Not difficult, "you cannot adjucate some value other than none" might be clearer. I'll edit my post.
In a sense I'm more confused. Deleterious Circumstances (target is a man) would feel perfectly reasonable as a minor flaw and covers the idea of guy who is bad at something. Mechanically I think it might even work with the @TheFoggyOldLamp's Myopic Talent to cover someone guy who can only (reasonably) do X since roughly half of the targets you might want to use Corpus against should be female.
One thing that comes to mind is that while I don't completely agree with it I generally appreciate the idea that taking a flaw is a request to the SG to make the flaw count. Specifically if the flaw is one of the ones that is generically somewhat weak the SG is well within the realm of good play to make it show up and cause at least some issues.
I can appreciate that the flaw does stand on the toes of Deficient Tech/Form, and the way in which I present it doesn't quite describe my intent for. But ultimately I do think their utilities differ in terms of cross-section and so make either viable choices.
Deficient Form flat-out halves all the totals that feature a particular form put to any purpose. This is pretty huge considering how often requisites come about, but you can tend to play around it. "I'm not using wind to fly, I'm using Re:Te on my big stompy sabbatons!"
Myopic Talent (minor) is the same in terms of its liability, but different in that approaches the same problem by attacking purpose as opposed to means. "Can I use Pe:He(Te) to destroy this crossbow bolt in my grog and help him recover?" Well unfortunately you have Myopic Talent (Fruiting) so unless you want to turn the crossbow bolt into quite a long [fruit that isn't anachronistic] and then destroy that, your totals are too low to do it.
You can see how it presents a similar complication to play with a similar opportunity to work around it but the solution for Myopic Talent can still occur within the specialization, whereas Deficient Form would require you to fully avoid using the form in which the complication begins. In either case, the limitation to play is simply the requirement that you become a little more quixotic in your thinking or else find help from someplace else.
That isn't to say that your issues aren't totally valid. The main problem with Myopic Talent is probably the fact that magical foci are on very shaky ground in terms of set theory.
For instance, one of the Major Foci is Damage, any Tech:Form combination that causes damage, which could potentially feature nearly every combination excluding Intellego. But its functional opposite, Healing, is a Minor Focus-- understandable considering healing is difficult under Hermetic Theory --but would it still be a minor focus if this were not the case? Moreover, Healing includes three forms and its only point of comparison is Self-Transformation, which uses two forms but makes it so that you have only one possible target.
This is not to say that magical foci are bad, quite the opposite. It's more to indicate that the difficulty with balancing something like Myopic Talent against something direct like Deleterious Circumstances or Deficient Form begins with the fact that magical foci are a little notionally diffuse. And, in fact, this is probably why something similar doesn't feature in published work.
I mean can you imagine having this in the core book and then introducing players to the Astrological Foci?
"I'll take a Myopic Talent in The Sun because I want to be absolutely shafted when I try to do anything that doesn't affect Presence, Stamina, rulership, kings, leadership, authority, health, and/or well-being."
Just had a horrible thought.
There are a couple of cannon Focii that encompass all Arts - mMF in Certamen, and MMF in Familairs jump to mind.