If My Hens Eat Buttercups (which Is Poisonous To Humans), Would A Human Have Stomach Problems If He/she Ate One Of The Hens Eggs?

No it will not cause any harm to your health as there are special enzymes present in hen's stomach which nullifies the poisonous effects of those things which are poisonous for human beings.It's naturally present in their stomach so it is save to eat hen which is fed on buttercups.