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Did you miss the bit where many people don't know they have photosensitive epilepsy until they have a seizure?
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True of many things. Most people don't know they have a nut or bee sting allergy until they have a (possibly fatal) reaction. However anyone who sets out to deliberately produce any serious & harmful reaction in someone else without their consent is guilty of a criminal offence regardless of whether the act in question is capable of producing such a reaction in that person.
If I shoot a person in the chest with a .22, I don't get off a charge of attempted murder because unknown to me the victim was wearing a kevlar jacket and so my act had in fact no chance of hurting them. Conversely If I shoot someone in the head with what I believed to be a harmless blank and the person dies because I was ignorant of the fact that the shockwave from a blank is just as deadly as a bullet at close range, I am not guilty of a crime.
As another pair of examples - if I buy talcum powder believing it to be heroin, I am guilty of a crime. But if I buy heroin believing it to be talcum powder, I am not guilty of a crime. Whether such a story is believed by a court is of course a different matter.
It is the intent, not the outcome that matters.