Will a veiled woman be held accountable if some of her hair unintentionally shows?

There is no blame upon you for what appears of your hair unintentionally. However, you must hasten to cover it immediately upon realizing it has appeared, according to the words of Allah Almighty: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended" (Quran 33:5). And the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned for my nation mistakes, forgetfulness, and what they are coerced into." As for if you are negligent in covering your hair or your 'awrah, then you will be held accountable for that, because it is among the forbidden acts.