I've got stinging nettles all over my yard this year. I never noticed them before, though I suppose they must have been here. Maybe this summer's hot, dry weather - coming after a cool, wet spring - was just perfect for them to grow quickly. I've heard of them before, certainly, but never knew exactly what they looked like.
Earlier this summer I plucked a weed from my pepper plants that I thought was galinsoga, but it stung me! At first I thought it must be an insect on the plant, but then it remained irritated, as if something was in the skin of my finger. Indeed, that is what happens - a piece of the "hair" breaks off in the skin, along with some chemicals, and it hurts! This plant is in the family Urticaceae.
At least for me, there is no long-lasting damage, but the irritation from the stings does last for a couple of hours. I would definitely recommend wearing gloves when handling the stuff.
The confusion with galinsoga is interesting - the flowers are completely different, but the one I saw first did not have flowers yet. The leaves are somewhat similar in shape, but the nettle leaves are much more serrated. Also, over time, the nettle plants get much, much bigger.