I'm considering the following homebrew flaw (or possibly virtue), but I'm not sure if it is a minor flaw or possibly even a minor virtue:
Flawed Experimenter
Either due to a flawed understanding of magic theory, a poor education or a lackadaisical attitude you cannot perform any laboratory activities without experimenting. Years of practice has however made you used to avoiding accidents in the lab; Should a roll on the extraordinary results chart proves unfavorable, you may choose to spend a confidence point to reroll the result. Doing so however increases the risk modifier by 1 for the next roll, but you do not gain an increase to your lab total. You may do so only once per laboratory activity and you may not exercise this option when doing original research.
Up to this point, it is a reasonable minor flaw that has no complexity.
The extra +1 risk is actually useful. Discovery is at 10 on the extraordinary table, as it requires a really rare roll, a 1 then a 5 (or exceptional risk), to get discovery. Your re-roll has increase by a factor of 10, the chance of getting discovery (a roll of 9). The best experiment option.
A re-roll option on something as chaotic as experimentation is amazing. This is a virtue in my opinion.
I see two balance issues. Where can the re-roll be done?; and the confidence point cost.
If the re-roll is only for the first part of the extraordinary results table, it is probably balanced. If the re-roll can be anywhere, overpowered.
I can see a character doing some near pointless lab activity with a +3 exceptional risk mod, fishing for discovery. Then on the discovery table, if they don't roll 7 plus on the dice to get 2 rolls, roll again, this time only needing 6 plus for 2 rolls.
If confidence points are given rarely, this virtue might be under powered. If the re-roll is limited to the first table, you might want to say as a virtue the confidence point cost goes away.
Prone to Chance (Minor Ordeal):
This ordeal changes the magus’s personality, making him extremely keen on risk taking and experimentation. After undergoing this Ordeal, he must experiment on every laboratory activity that allows experimentation, loving the thrill of the unknown and its unexpected outcomes.
He receives the Minor Ordeal Flaw Prone to Chance. This Ordeal is incompatible with the Ordeal Averse to Risk.
That supports the idea that "must experiment" (without the reroll option) is about right for a minor flaw.
Well, there is a downside: rerolling costs a Confidence point, and may only be attempted once per laboratory activity (which is not entirely clear - I guess it means that you can only do it once for any given "project", even if it takes multiple seasons to be completed).
Note that "you cannot perform any laboratory activity without experimenting" means that the magus cannot, for example, open items for enchantment or fix arcane connections in the laboratory - since those activities do not allow experimentation. I think writing "you must experiment on every laboratory activity that allows experimentation" would be a better choice.
Even with this fix, I think it's worth at least a minor Flaw. I say this fully aware that even without the reroll option it's a canonical Minor Ordeal Flaw (Prone to Chance, HoH:MC p.120). But Prone to Chance is not a well-balanced as a Minor Flaw at all in my view, being much closer to a Major Flaw. None in my troupe would ever willingly take it as a Minor Flaw for a PC, because it seriously jeopardizes a number crucial activities that should really avoid experimentation, such as binding (or enchanting the bond of) a familiar, or placing one more enchantment into a talisman. This problem is lessened, but persists, with the reroll option.
I would actually rewrite it stripping it of any good effect, but allowing no-experimentation labwork for a price (and I would make that price something other than Confidence, which may be too scarce or too abundant to be a good balancing factor). The following would be something that my troupe may occasionally want to take for PCs:
Prone to Chance Minor Flaw, Hermetic
Your laboratory work is prone to unexpected outcomes. This may be due to an unconventional understanding of magic theory, a careless or incurably curious attitude, or some supernatural agency. You effectively experiment in any season of laboratory work in which you could experiment. You can negate this Flaw for any one season of work through extraordinary focus and meticulous attention, spending two seasons instead of one.
I think including the option to reroll the result with a confidence point completely makes this a virtue. For a flaw, see Exciting Experimentation instead, where the storyteller picks which of the two results is more fun.
One of my players effectively has this flaw (must experiment in the lab, except when following lab texts). It seems to fit the general level of nuisance of a minor flaw, having so far created (i.e. allowed me to run) one story and wasted one season. Allowing the reroll with a confidence point turns it at least neutral. It's probably best to consider that part an entirely separate virtue.