Last summer a border collie chewed on some water hemlock near Fort Collins and died on its way to the Colorado State University Veterinary Teaching Hospital.
As every plant guide will tell you, water hemlock in bloom looks a lot like Queen Anne's lace, which is completely harmless — its roots are edible. So you can have a fruitful wildcrafting experience, or you can die twitching and drooling.
How do you tell the difference? I was taught the purple blotches, but there are other distinctions as well, which this well-illustrated web page explains.
You might not think of water hemlock as attractive to dogs, but some will chew on plants randomly. (I'm looking at you, Fisher.)