Spotted water hemlock

"The confusion with parsnips can be fatal as C. maculata is extremely poisonous. It is considered to be North America's most toxic native plant."

"The chief poison is cicutoxin, an unsaturated aliphatic alcohol that is most concentrated in the roots. Upon human consumption, nausea, vomiting, and tremors occur within 30–60 minutes, followed by severe cramps, projectile vomiting, and convulsions."

It supposedly tastes good though.